For match mode, you can choose at most 27 heroes of the same class.

The minimum requirement for not losing you ranking point:

Silver: 3 matches in 2 days Gold: 4 matches in 2 days Platinum: 5 matches in 2 days Diamond: 6 matches in 2 days Master and above: 8 matches in 2 days

(There will be a ranking status bar in game. If it’s red, then you don’t have played enough games. If it’s green, you will not lose ranking points)