How many smartphones can I take on an airplane?

You’re overlooking something huge.

You need the phones to do your job. To engage in employment.

Some countries have a big problem with that, depending on your visa.

It comes down to that country’s treatment of digital nomads. That’s when you’re an American working for a USA company, your desk, manager and division are in Chicago, you deal with US customers, get a USD-denominated paycheck with a US address wired into a US bank… and you’re surfing the WiFi at a Tim Horton’s in Canada.

By example, I hear Canada is happy with digital nomads, and UK is not. Say “need em for work” at the wrong customs desk and they’ll cancel your visa and send you home.

I am going to give you a different take on this question. It is in your question (customs) and seems to be overlooked in most answers.

It is not really about planes, batteries, safety, or how big your bag is, it is mostly about the law of the departure and destination countries (not usually transit countries). Many countries prohibit what you can bring in to a country so yes it differs by country. Not only this but I know in Latin America that virtually no country exists that would allow you to bring this much (not just technically but actually legally).

My expertise is Ecuador.

Ecuadorian limits on bringing electronics into Ecuador are strict and enforced and you could not bring this many items in, and your carry on and regular bags are scanned on arrival.

From Ecuador Customs Sheet.

Travelers or head of household may enter up to two (2) additional
units, one (1) new and one (1) used, of the following portable
articles:

Photographic camera, video camera, mobile telephone, electronic
agenda, portable or non portable video game, electronic calculator,
portable computer and its accessories (mouse, headphone, keyboard and
other).

Also, all travelers or head of household may enter up to one
unit (1) new or used, of the following:

Portable image reproducer,
sound or video player, portable television up to 21 inches. Desktop
and its accessories (mouse, headphones, camera, keyboard and other).
Prismatic equipment, projector, monitor up to 21 inches and telephone,
printer or fax.

I think laptops will be considered portable computers and electronic agendas are those digital electronic organizer things business-like people used to use, I think they were called Palms.

I am a software engineer and I also test the software on many different smartphones and tablets. I fly frequently within Europe, so my answer will be limited to this area. However, this is also from west to east and east to west, outside of the Schengen region! As a Dutch citizen, I rarely need a visa to travel, which makes this easy.

I do carry a lot of devices with me; most of which either on my person or in my hand luggage. Airport security does give you a strange look when you put more than ten devices in your box to pass security, but I’ve never had any questions about it. In fact, my entire hand luggage often consists of an extra sweater or hoodie, and the rest is electronics.

Just to be on the safe side of things, I do carry the paperwork to prove I am a software engineer and not a salesman. If customs ever ask about the amount of devices, they will probably do so because they think you’re going to sell the devices to avoid taxes; to turn a bigger profit. I would highly doubt they would ever deem it a security issue.

So in short, carry them with you, pretend all is normal (because it is!) and if they ask questions, answer honestly (probably always a good idea). If you travel between countries that have very different rules about taxing, do also carry a proof of employment, just to be on the safe side of things and to avoid a lot of hassle.

There are three concerns: 1) terrorism; 2) export/import limitations; and 3) the fire hazard from the lithium contained in the cell phone and laptop batteries.

1. Terrorism: the difficulty of addressing terrorism is that the level of screening and scrutiny is often arbitrary depending on the current political climate and the capriciousness of the security agent screening your baggage.

2. Export/import limitations: In most cases, you need to prove that you are carrying the devices for your own personal use and not for resale. As long as the devices are unboxed and in ‘used’ condition, this shouldn’t be difficult.

3. Fire hazard: I’ll mainly address the latter concern regarding the FAA rules on fire hazard.

FAA regulations do not limit the quantity of small lithium cells/batteries carried on board by passengers although they do limit the maximum size (capacity) of larger cells/batteries – such as those used in power wheelchairs and high-powered camera equipment (studio flashes).

However, the batteries must be enclosed in the devices and the devices must be for personal use (and not, for example, for resale) and so you might have trouble convincing the pilot/TSA/flight attendants that you have (say) ten phones for truly personal use.

But five cellphones is actually pretty reasonable by international standards as you might need to swap sims between different phones as you travel between different countries.

49 CFR 175.10(a)(18)

(A) The battery must be removed from the wheelchair or other mobility
aid according to instructions provided by the wheelchair or other
mobility aid owner or its manufacturer;

(B) The battery must be carried in carry-on baggage only;

(C) Battery terminals must be protected from short circuits (by
placement in original retail packaging or otherwise insulating the
terminal e.g. by taping over exposed terminals or placing each battery
in a separate plastic bag or protective pouch);

(D) The battery must not exceed 300 Watt-hour (Wh); and

(E) A maximum of one spare battery not exceeding 300 Wh or two spares
not exceeding 160 Wh each may be carried;

(vi) The pilot-in-command is advised either orally or in writing,
prior to departure, as to the location of the lithium ion battery or
batteries aboard the aircraft.

(18) Except as provided in §173.21 of this subchapter, portable
electronic devices (e.g., watches, calculating machines, cameras,
cellular phones, laptop and notebook computers, camcorders, medical
devices etc.) containing dry cells or dry batteries (including lithium
cells or batteries) and spare dry cells or batteries for these
devices, when carried by passengers or crew members for personal use.
Portable electronic devices powered by lithium batteries may be
carried in either checked or carry-on baggage. Spare lithium batteries
must be carried in carry-on baggage only. Each installed or spare
lithium battery must be of a type proven to meet the requirements of
each test in the UN Manual of Tests and Criteria, Part III,
Sub-section 38.3 and each spare lithium battery must be individually
protected so as to prevent short circuits (e.g., by placement in
original retail packaging, by otherwise insulating terminals by taping
over exposed terminals, or placing each battery in a separate plastic
bag or protective pouch). In addition, each installed or spare lithium
battery must not exceed the following:

(i) For a lithium metal battery, a lithium content of not more than 2
grams per battery; or

(ii) For a lithium ion battery, the Watt-hour rating must not exceed
100 Wh. With the approval of the operator, portable electronic devices
may contain lithium ion batteries exceeding 100 Wh, but not exceeding
160 Wh and no more than two individually protected lithium ion
batteries each exceeding 100 Wh, but not exceeding 160 Wh, may be
carried per person as spare batteries in carry-on baggage.

(iii) For a non-spillable battery, the battery and equipment must
conform to §173.159a(d). Each battery must not exceed a voltage
greater than 12 volts and a watt-hour rating of not more than 100 Wh.
No more than two individually protected spare batteries may be
carried. Such equipment and spare batteries must be carried in checked
or carry-on baggage.

(iv) Articles containing lithium metal or lithium ion cells or
batteries the primary purpose of which is to provide power to another
device must be carried as spare batteries in accordance with the
provisions of this paragraph.

(19) Except as provided in §173.21 of this subchapter, battery-powered
portable electronic smoking devices (e.g., e-cigarettes, e-cigs,
e-cigars, e-pipes, e-hookahs, personal vaporizers, electronic nicotine
delivery systems) when carried by passengers or crewmembers for
personal use must be carried on one’s person or in carry-on baggage
only. Spare lithium batteries must be individually protected so as to
prevent short circuits (by placement in original retail packaging or
by otherwise insulating terminals, e.g., by taping over exposed
terminals or placing each battery in a separate plastic bag or
protective pouch). Each lithium battery must be of a type which meets
the requirements of each test in the UN Manual of Tests and Criteria,
Part III, Sub-section 38.3. Recharging of the devices and/or the
batteries on board the aircraft is not permitted. Each battery must
not exceed the following:

(i) For lithium metal batteries, a lithium content of 2 grams; or

(ii) For lithium ion batteries, a Watt-hour rating of 100 Wh.

The IATA has similar regulations with similar restrictions:

2.3.5.9 Portable Electronic Devices (including medical devices) containing Batteries
2.3.5.9.1 Portable electronic devices (including medical devices) (such as watches, calculating machines, cameras, cellular phones,
lap-top computers, camcorders, etc.) containing batteries when carried
by passengers or crew for personal use, which should
be carried in carry-on baggage.
(APCS/Cargo 15/12/2014 IATA Lithium Battery Guidance Document – 2014 Page 12)