I think a woman can have as many guys in her as she wants, and say stop at any time. As long as that's done, I'd say it's fine. It's not the number, but how many she wants to handle.
Hell, my wife could easily handle a half dozen now that she's in her sexual prime. Luckily for me she says they'd all have to be clones of me. Unlucky for me is that fact that I'm far, far from my sexual prime.