You need 14 tiles to win Mahjong Island. When all 14 match one line on the Island Card exactly, you Win.
What do the 14 tiles look like?
Each hand on the card is written as groups of identical tiles. A group can be a Single (1 tile), a Pair (2), a Set (3), or a Quad (4). It can also be a Quint (5) or a Sextet (6). Some lines also ask for numbers in a row, like 1-2-3-4-5, as Singles.
Every line on the card adds up to 14 tiles. Your job is to collect the exact groups that one line asks for. You only need to complete one line to Win.
Can a Joker count as one of the 14?
Yes, a Joker can stand in for one tile in a Set, Quad, Quint, or Sextet. It can never fill a Single or a Pair. To learn more, read the rules for Mahjong Island Jokers.
Where does the 14th tile come from?
Each player is dealt 13 tiles. The dealer gets a 14th tile and discards first, once the Charleston (the passing of tiles before play) is done. On your turn, you draw a tile from the Draw Pile, which brings you to 14. Then you discard 1 tile, unless your hand is complete and you Win.
You can Win on a tile you draw yourself, called a Self-Drawn Win. You can also Win with Take Tile, picking up another player's discard when it completes your hand.
To see the hands you can build, read Mahjong Island Winning Hands: Every Hand With Examples. For every rule in one place, see Mahjong Island Rules: How to Win a Hand.