Title: Mexican Train
Content: 2–4 players: 10 dominoes each ; 5–6 players:  9 dominoes each ; 7–8 players:   7 dominoes each 2–4 players: 15 dominoes each ; 5–6 players:  12 dominoes each ; 7–8 players:   10 dominoes each At the start of each round, the dominoes are placed face side down, shuffled, and drawn by players to form their hands. Using the double-nine set, players draw dominoes as follows: Using the double-twelve set, players draw dominoes as follows: Any remaining dominoes are placed to one side, forming the "boneyard".
Title: Mexican Train
Content: Double-six (2 players) ; Double-fifteen (9–12 players) ; Double-eighteen (13 players) One token or marker for each player ; A special spacer, known as the "station" or "hub", used to evenly space the trains around the central domino (optional). ; Pencil and paper to keep score A double-twelve set of dominoes is marketed as the standard for Mexican Train, and accommodates up to eight players. Other sets are commonly used as well. The following alternate sets are common, depending on the number of people playing: In addition to dominoes, the game also requires: Many sets of dominoes include a station and special train-shaped tokens for markers, and packaged games with a central "station" and custom tokens are available. However, the station piece is not strictly needed, and anything from coins to poker chips to even pieces of candy or slips of paper can be used as markers.
Title: Mexican Train
Content:  There are a number of other versions of the rules for Mexican Train, varying the number of dominoes drawn by each player, the rules for playing doubles, or the number of tiles that can be played during one turn. For example:
Title: Mexican Train
Content:  The object of all trains games is to be the first player to place all of their dominoes. Dominoes may be placed onto the player's train, onto the Mexican train if available, or on the trains of other players under special circumstances. The middle tile counts as the start of a player's multiple tile train.
Title: Mexican Train
Content:  Mexican Train is a game played with dominoes. The object of the game is for a player to play all the tiles from his or her hand onto one or more chains, or trains, emanating from a central hub or "station". The game's most popular name comes from a special optional train that belongs to all players. However, the game can be played without the Mexican train; such variants are generally called "private trains" or "domino trains". It is related to the game Chicken Foot.
Title: Mexican Train
Content:  With a standard double-twelve set the double twelve is placed in the station. In each successive round the next lower double is used until all doubles are used. The double-blank is the final round. Play continues to the left. Each person lays one legally placed domino per turn, or two if the player's first domino is a double. If they are unable to, they must draw a domino from the boneyard. If they are able to lay that domino, they must do so immediately. Otherwise, their turn is over and play continues to the left, each player trying to place all their dominoes by playing matching dominoes one at a time, end to end. A train can be as long as the players can make it; it only ends when all dominoes that could match its endpoint have already been played. As a result, trains can become quite long, especially with an extended domino set. It is acceptable to "bend" the train 90° or 180° to keep the train on the playing surface, as long as it does not interfere with the endpoints of other trains.
Title: List of domino games
Content:  This member of the trains family of games, similar to the basic version of trains but with an additional Mexican train, is played mainly in the United States. It is typically played by at least four players using at least a double-twelve set. The game starts with a double in the middle of the table, acting as a spinner from which the players' "private trains" branch off. An additional Mexican train, initially of length zero, also starts from the central spinner. Unless the tile played is a double, only one tile can be played per turn. Tiles must normally be played to the player's own train or the Mexican train. A player who 
Title: Mexican Train
Content:  The Mexican train is an additional train that anyone may play on during their turn. They can start the train by playing a domino matching the engine (i.e. the double played at the beginning of the round) or add to the train.
Title: Mexican Train
Content:  Similar to the fast game variation listed above, instead of simultaneously, each person takes turns playing as many dominoes on their train as possible. Any player that is unable to play on their first turn draws a domino, and may then begin their first turn or pass. When a player who has passed finally starts their train, they may play as many dominoes as they can string together. After starting their train, players are only permitted to play one domino at a time as usual, unless playing a double. This version allows a player to save their "first turn" string of dominoes for when they have a chance, instead of being stuck with all dominoes from the beginning.
Title: Mexican Train
Content:  When a double is played, it is placed perpendicular to the train. Subsequently, adding a domino to the double is called "finishing", "satisfying the double" or "covering the double." The player that plays the double has to cover the double or their train becomes public and someone else has to cover the double before normal play can resume. If a double remains unfinished after it has been played, the train becomes a "public" train. No other train can be added to by any player until someone "finishes the double". Play passes to the next player who can legally play on this train (if the train is theirs, or if they can legally play on "public" trains). If they cannot "finish the double", they must draw a domino, and if they cannot play it on the double, their own train becomes "public". Single and double blanks are considered wilds. Mexican train dominoes can end on a double. Branching on doubles as in Chicken Foot is allowed as an option (see Variations below).

Question: how many dominoes do you need for mexican train?
Answer: A double-twelve set.