Okay, well the maths works like this - you have to consider these things:

- How many nappy changes per 24 hours.
- How often you're going to wash.
- The weather.

Baby BeeHinds fitteds (the bamboo or hemp ones that need a cover) are a great one size nappy! They can be pretty bulky on a newborn though. River was born 9.4lb (4.2kg), and they fit him from when he was about 2 weeks? He still wears them now (17 months) and they're quite a trim nappy now!
As a newborn, we did about 8 nappy changes per 24 hours I think?

We washed every second day. Or third...we had quite a few nappies - flat flannel nappies, and prefolds, and fitteds with covers - as well as the odd disposable here and there.
So that would be 8 x 3 days. So, 24.

Except - that if it's really rainy, unless you tumble dry a lot (not great for elastic to do it ALL the time) bamboo can take awhile to dry, about a whole day on the line if it's cloudy - though you could line or clothehorse dry and then finish them off in a tumble dryer.
(This is where flats and prefolds come in handy, they dry quickly)

Usually, for people trying new nappies, I'd say - just buy a few and see if you like them/how they suit your baby - and then if you like them, get some more.

However, with Baby BeeHinds, being one size - if you find them too bulky on your newborn, you can always try them again a few weeks down the track. They really are quite a good 'workhorse' nappy.

Now that Riv's older, we have fewer nappy changes per day, and wash about every 2 days. But - we don't use as many fitteds with covers - fitteds with well fitting covers are great for that explodey poo! But once Riv was on solids, we started to use more Pocket nappies or All-In-Ones.

There's a Help for New Users thread here that might help answer any other questions you have too?
All the best, modern cloth nappies involve a huge learning curve!