how many?

2 is max in 2on2 but you no have limit in 3on3 and 5on5

It depends from the ladder you're signed up to. There are 1on1's, 2on2's, 3v3's and 5v5's... In the team you can have as many people as you want, but there aren't ladders that allow to play in more than 5v5.