I'm not sure how many you'd need for 2, (am expecting my second child and need to get more myself) but for 1 I've got about 16 that I add to with terries when ds is weeing lots or I'm behind with the washing
... I'm wondering about getting another 10 or 15 M, L or OS, plus some smalls for the first few months. Your littlie won't really fit in the nappies for your toddler for a while (although I started ds in one size nappies really early as we can't afford to buy each size, and his cloth bum was huge!) so getting some smaller nappies for them to start might be a good idea.
I know that wahms who use fleece or do embroidery will have car nappies, and think I've seen some on Snuggleblanks (they're on fb) if you wanted to look there? There are people on here with a much better idea than me where to get pretty nappies though - all mine are fairly generic so I'm not that sure where might have them!
Good luck getting the nappy stash underway