I'm a bit embarrassed that I found this question so quickly...I'm not obsessed with this, I just happened to be passing through BB before wrapping some Christmas presents - I SWEAR!

How many nappies do most people use with a newborn?

Umm, they say to allow for about 12 nappies a day? So you'll want at least 24, which sees you washing daily. (Hmm, now I think I might get another 12...so I can have a lazy day here and there!) It's not hard though. I'd be suggesting 24 flats or prefolds, if you were going to go down that route, and then a couple of fitted nappies for when it's 3 am and you're still half asleep.

Do OSFA nappies really fit newborns?

Depends how big they are, Riv was 9.4lb and just fit Baby BeeHinds fitted nappies. I think I first used a BumGenius when he was around 3/4 weeks and that fit fine. I depends on how much bulk you're happy with - they can look HUGE on a tiny widdle newborn!

For fitteds, apparently Whizzkids newborns are great, well priced too! But can be hard to nab. I think, if funds allow, I'll get a few bamboo nappies, like Tots Bots Bamboozles in size 1 (or Bambeino Mios, because I prefer to use a snappi, I think it gives a better fit) which are bamboo terry, so quite trim, and I know it took River aaaages to grow out of his Tots Bots size 1's! Tots Bots Fluffles might not be a bad option either, they're a synthetic nappy (microfibre), so the occasional child might react, but they're much faster drying than bamboo.
Sorry, that was a bit of a tangent on fitteds hey.

Also what is the difference between prefold and flats?

A flat is your traditional square of flannel or terry towelling that our mums and grandmas used. Flannel is trimmer, and terry can be more absorbent.
Prefolds are smaller squares, with a couple of layers of material. They're usually 3 panels, with the middle panel having more layers. If you type 'prefolds' in Google Images, you should come up with a few pics to give you an idea.

You'll need to pin flannel (as a snappi won't 'grab'), but you can snappi terry and prefolds. (Google Image 'snappi' if you don't know what those are too!) I love prefolds, find them really easy to use, and wash, but they still dry relatively quickly - though not much beats a flat for drying time!
We had flannel flats, but I was rubbish at pinning them - dh was a pro though, and many people love them.

There you go, others might pop in with other ideas for nb fitteds, and what OSFA's fitted their newborns! I'm off to wrap my presents!