How much space is there on the battlefield?
The legendary neutral minion card Onyxia has the following text:
Battlecry: Summon 1/1 Welps until your side of the battlefield is full.
So how much space is there on the battlefield? When will it be considered "full"?
The maximum number of minions per side is 7.
source: http://hearthstone.gamepedia.com/Minion