6 teams play each other once how may games they need to play

Well, generally speaking, they can play as much as they want, but if you want to get every team to play against another team exactly once, they each need to play 5 games, resulting in a total of 5*(6/2) = 15 games. (each team needs to play against the other 5 teams).

For a single elimination, 5 games are enough.

It all depends on the restrictions here.