Ninja is a coordination game/fighting game for almost any number of players (but is usually most fun with 5 to 15 players). You need a small playing field without obstacles (but that can be a room indoors as well).
You don't need any equipment to play Ninja.
The players are standing in a cicle (close together). Upon the starting signal, they jump outwards, with a typical "Ninja scream". They also make some Ninja moves and stay in that attack position.
Then, one after the other of the players may attack. The attacker choses any other player (but usually, the immediate neighbours are easiest) and attacks this player by hitting his hand. This has to be done in just one ninja attack move. Of course, the other player may defend by removing his hand, stepping aside, jumping away, etc. When the attack is successful (the hand is hit), then the defender has to take his hand out of the game (and put it on his back). There are no further consequences if the attack is not successful. After the attack/defend move, both players keep their last position and may not move unless they are attacked.
After an attack, the next player (clockwise around the circle) may attack.
Any player having lost both of his hands is out of the game. The last player to remain in the game has won.