I'm not really good at guessing how many you'd need...that's why I have 40+ nappies. That said, half my stash are on the line in the rain and even with drying time of the rest I would be fine to exist on what I have.

What shape is your little guy?
Nate is looong and skinny.
We have
Monkey Doodlez
BumGs
Ecobubs
Itti Bittis (not so great on him)
BBHs have hemp and bamboo - hate the hemp
ctue tooshies
cushie tooshies
tots bots fitteds
bundles of buns (our fave - i have 7)
berry plush - a useless fit on him

hmmm I think that's all.
Sadly it's trial and error most of the time. Look for ones with good reviews. Lots of the girls sell them and can't recomend them highly enough.