There are currently 15 international plug types that are supported across 250 cities and countries. Each of these 15 plug types is assigned a letter by the US Department of Commerce International Trade Administration (ITA).
Type C plugs are the most widely supported - by about 143 cities and countries worldwide. Type O plugs are the least supported - only by 1 country (Thailand). Here are the list of international plug types and cities and countries that support them - a useful guide for international travelers and tourists.