Apparently 10... Sanderson has said 2 sets of a 5-book story arc. He does seem to have slowed down in his writing, so that may be a while.
Yeah. He has so many projects on his plate. After Stormlight 4, I think he is going to finish Era 2 of Mistborn. There is a Warbreaker sequel up soon (important for Stormlight Archive). And his sci fi series Starlight is due a final book.