How do you know when you're done having kids?

We've always said 3. Both babies have been birth control babies. Our kids will be 15 months apart. I'm just wondering, if you're done having kids, how did you know? Did you feel complete?

Our baby girl isn't here yet, but I feel like 2 isn't enough. But on the other hand, I haven't finished school, I live with my in laws, I haven't gotten even settled with a career. We're definitely not hurting financially, but I wouldn't say we're where we'd like to be. So we've thought of only doing 2 kids so we can focus on providing for them. I know things might be different once she's here, but my heart feels like 2 isn't enough.