Q: What is Illya’s capacity as a Grail? Hypothetically, if the Einzbern won the War, Illya absorbed the other 6 Servants besides Berserker, and had some shred of human will left, could she still don the Dress of Heaven and initiate the Grail ritual?

A: That’s what Liz and Sella are for. Illya can only take up to 4 - I would think that absorbing a fifth Servant wouldn’t allow her to function as a human any longer.

Fate/complete material III; FAQ, pg. 131