I'd guess they'll keep going until they see diminishing returns. PG has said y-combinator is an effort to mass produce the startup (don't have the direct quote

I'd guess they'll keep going until they see diminishing returns. PG has said y-combinator is an effort to mass produce the startup (don't have the direct quote offhand). With that in mind, they seem to be looking to optimize the entire experience rather than just seeking to maximize profit. I imagine they'll let it grow only to the point that they still feel like they can work with each of the startups in some capacity (and within reason).