3v3 is the biggest they will support at launch. According to their development road map they will be supporting huge maps with up to 12 players sometime during the third quarter of this year. I don't know if they will increase the number incrementally over the months or go straight from 6 to 12.
You can see the road map here:
http://steamcommunity.com/app/228880/discussions/1/490123938434377747/
