I'd maybe go upto 30. Means you dont have to wash everyday. I'd personally have 40 smalls, and wouldn't have any newborn as both of mine we're over 8lbs. Smalls, although a teeny bit big, fit well straight off. Oh, we had Sandy's so not posh nappies
When T was tiny I only had about 15 that worked (I had loads of others but they didnt fit right!) I found that fine because I tumbled anything that could be safely tumbled til I bought some more of what worked.