There's no limit. Or rather, the only limit is your other investors. If you'd already taken a series A round from a VC fund, they probably wouldn't like the idea.
Why do you think VCs wouldn't like it?
Dilution, for one thing. Assuming standard YC terms were offered, the stock allocation and valuation would be incompatible with a VC A round.
AirBnB was quite established before they got into YC.