ya know, negotiate.
I married wanting 0, husband wanted 2. You'd think we would have talked about that. Well, we did, but we both figured we could change each others mind.
It took him about 7 years to talk me into 1 (then longer to actually have the 1). Then, we were happy and done.
I have friends where one of the members wanted more than the other. If the person who wanted to stop really wanted to stop, then they won. I mean, you just get something permanent done. If the woman is done it's easier, because you can go on birth control. For the man it's a little harder without the snip.
Generally it defaults to the person who wants less. Otherwise, I've heard stories of women wanting more and having them, and the dad just saying "well, you wanted it, it's your job". Which, um, no.
Long story short, we have two. But in my defense, it was kid #1 who was begging for a sibling. Tried, failed, gave up, and whoops.