Replaying Yogg-Saron

Yogg-Saron reads:

Cast a random spell for each spell you've cast this game (targets chosen randomly).

My question is: Let's say I've cast 10 spells this game. I now play Yogg-Saron, and he will cast 10 spells for me. The last spell he casts is a Shadowstep/Vanish (something that returns him to my hand).

If I play him next turn, how many spells will he cast for me? Still 10 (because I as a player did not cast any more), or 20 (because Yogg reads 'cast')?