If it is 2 against McCarthy, McCarthy wins by splitting the votes. I hope it will be 1 against 1.
Speaker is elected by the whole House and must get a majority. A group larger than the vote difference between the two parties can prevent ANY Speaker being elected by voting for a third or many other candidates. If they make a credible threat in the caucus, they can extract concessions or force a compromise choice.
Actually, I’m not sure that’s true. I’m pretty sure that someone has to get 50%.