They way I'd look at it, is view it as if you're going into this planning on keeping every single baby. That way you've planned the expenses, the time, and the supplies that will be needed in that scenario. Then if you can't sell them all, you're covered. If you can, then great now your ready to breed again.. If you always just prepare for the highest extreme, then you're ready for the lowest (you sell all of them relatively soon) up until the highest, being that you have to keep em all.
Kind of like when I play poker. I just go into it thinking that I'm not going to win any money and the money that Im putting in is money that I'm "spending" to have fun. Then if I lose it all, no big deal. If I win some, it's a bonus.