How many dominoes do you need to play Mexican train?

Every set of dominoes includes all possible combinations of two numbers, from zero (blank) up to the highest number of pips in the set (for example, 12 in a double-12 set), as well as a double for each suit. Each combination of pips occurs only once in a set. A standard double-6 domino set consists of 28 tiles: 7 doubles and 21 singles.