There are multiple different kinds of domino sets, but the most common commercial sets are double six, with 28 tiles, and double nine, which has 55 tiles. Smaller and larger sets than these are also available.
A mathematical formula determines how many tiles are in a set: ((n+1)(n+2))/2, where n is the highest number found on a tile. There are many different ways to play the game of dominoes, and the number of tiles in the set usually determines which games are best suited for that set. The double-six set, for example, is most commonly used to play a blocking game with two players.