Upvote:7
Passportinfoguide has a surprisingly vague answer, but it does appear that it's about as specific as you might get, as many countries' embassies don't provide this information. From the page:
There is no short answer that applies to every traveler. The requirement varies from country to country. If you have (4) blank pages then you are probably fine. If you have less than that it may be an issue. It depends on the country you are visiting that will determine how many blank pages you actually need. Most countries will require that your passport have two (2) blank pages. Some countries require up to four (4) blank visa pages. Some airlines will not allow you to board the plane if this requirement in not met. If you are on a connecting flight to a departure city check with the airlines because sometimes they will not even let you board the flight to your departure city if you passport doesn't have enough blank pages.
Another resource you can use to calculate requirements, if you're visiting several countries, is the Gallery of passport stamps. This at least shows you which countries use a small stamp, versus an entire page.
If I find out more info I'll add it in here, but at present this seems about as accurate as one might get.