There are lots of different types of sets of dominoes although the most popular in the UK are double 6 dominoes.
Double 6 Dominoes
Double six dominoes comprise 28 tiles starting from double blank and featuring each combination of blank, and the numbers 1 , 2 , 3 , 4 , 5 through to double 6
Dominoes were originally Chinese. Chinese domino sets didn't feature the blank. They started with 21 tiles featuring all combinations of the numbers 1 to 6 and then had 11 duplicated tiles. Chinese dominoes were black with red and white dots and were also split into two "suits": originally called "Chinese" and "barbarian" and subsequently changed to "military" and "civilian"
Larger Domino Sets
There are larger dominoes sets available to allow more people to play at a time. The most common of these is double 9 dominoes, which feature 55 dominoes, one with each combination of blank, 1,2,3,4,5,6,7,8, and 9
There are also larger sets again of double twelve, double fifteen, double eighteen and even double twenty one dominoes which have 253 tiles.