I've been pondering on that a lot lately.
I have 2 kids, the youngest is 6 months old. Two of my closest friends are now pregnant, both with their 3rd child. I am delighted for them but recently I have felt a little pang when meeting them, talking about scans, kicks, buying things for the baby... And I thought I was broody, but I don't think I am anymore, I am just a bit sad that all that is all behind me. I loved being pregnant, loved giving birth (to my 2nd, if I had had internet access when pg with my 1st I would have had a much much better experience), I love the newborn days and all baby related stuff. I am a bit nostalgic of the excitement, the BFP, the baby kicking, the birth, the discovering your baby just after he/she was born, the telling people etc... But another child? No, I don't really want one. I am 31, so I have time, and OH is 40 so not too old either, but I see my 2 children and am happy. So is H.
I think any woman can have 12 children and still feel sad when they are finished.
I hope this made sense, hard to put how I feel into words.