Upvote:2
There are typical no limits on the amount of any electronics.
However, there are often size and weight limits, which implicitly limits your electronics count; and there are potential customs issues in the arrival country (but that's not really a flight limitation, but an immigration limitation).
In other words, if you manage to pack 100 iPhones in your carry-on and it weights less than 8 kg or whatever the limit is on that flight, the airline doesn't care.
Cost for excess luggage is specific to the airline, your booking class, and your status; there is no general answer.