Whenever this puzzlement about folks who say they "can't afford more kids" comes up, it always seems to be from parents of very young children, in situations where there is a sahp. And I can understand that. When you've got babies and toddlers, you're so caught up in that world, and you know very well that - barring unforeseen medical conditions - babies and toddlers don't cost a family very much.
Unless both parents have to/want to work outside the home, of course, in which case, they do.
But I don't hear that kind of "puzzlement" from parents of older kids. Those folks know that shoes, clothes, sports, field trips, music lessons, food all add up. And once your kid gets to be 7 or 8, it's a lot harder to find all those barely worn thrift store clothes you could always score for the little ones.
But all this aside, I think it really comes down to what you want for your kids and what you think they need. That's going to be highly individual for each family. Technically, we could have another two kids. Sure, we could. We have the space for them in the house, and we could afford to feed them and clothe them.
But we couldn't save for college for four the way we can for the two we have. And saving for college is one of our highest financial priorities. It obviously isn't for everyone, but it's a deal-breaker for us. We can't afford more kids.
But for me, finances are almost beside the point. The main way we can't afford more kids is emotionally. Two are the most our marriage, our patience, and our mental health can stand. And I think that a lot of people who say they can't afford more kids mean it in more ways than financially.