I doubt very much that Ripley had an exact idea or was even trying to like calculate in her head how many aliens would be waiting for her. I feel like it doesn't really matter to her, since she considered even just one of those things a major threat. So I doubt it made much of a difference to her if there were even 5, 10, 20 waiting for her etc.
Even if there were a hundred in there, I fully believe she would have still tried to save Newt, not caring if she died trying.