Don't invest too much in lots of xs and s nappies.....Grub had grown out of newborn sized ones by 3 weeks and smalls by 5 weeks (he was 7lbs 12oz born so not a massive baby either). If you can find some going cheap on the classifieds then get more but I wouldn't spend too much on the smaller sizes.
Mediums I'd have more of as they're going to last baby for longer.
It also depends on how often you're going to wash them...if it's every other day then I'd say 10-15 nappies is more than enough.
You can use things like muslins in the early days too