The Elder God N'Zoth's battlecry says:
"summon your deathrattle minions that died this game"
If this summons more minions than will fit on your side of the board, how does the game decide which minions will be summoned and which will not?
Minions that N'zoth resurrects will be random
Source: https://twitter.com/PlayHearthstone/status/713055841562206208