Hey Catriona. I really should be asleep, but hey, gotta live a little. :P

1) Depends how often you wash! I have...um...a lot of BBH's, maybe 11 Tots Bots (that fit a nb), at least 6 BumG's, prefolds, flats, a couple of other bits and pieces...and I often *just* get a nappy on Si's butt, because I wash when the moon is blue and jupiter aligns with saturn and the wind is blowing from the west (ie, I'm very lazy).
You might find yourself a bit short of nappies in those first days/weeks depending on how big bub is, and how they're shaped. My #1 was easy to nappy as a nb, chunky thighs, #2 had skinny chicken legs til about 3-4 weeks, so the flats/prefolds were tricky before then, and the BBH's seemed too bulky til about 5 weeks. So we supplemented with sposies in the first weeks. It's worth considering anyway in those first weeks, til you get back on top of washing.
You will quickly figure out if you need more nappies. And if a nappy doesn't fit, try it again the next week, that's how fast babies grow.

2) Kinda the same, depends how much you wash. 4-6 covers sounds good. You can always rinse PUL covers if you have some in the wash and need a clean one really soon. For the nb stage I love covers with gussets (like Tots Bots), allows for a snugger fit at the legs.

3) You can use pins for flannelettes, or just tuck them (like you do with a towel after a shower). Your terries can be pinned, tucked or snappi'ed. You might find the terries a bit bulky on a small baby til a few weeks.

4) I dry pail everything. I'm learning that the thing about a wet weather baby is that stains are taking slightly longer to vanish due to less sun. I still dry pail though, most stains are gone, and the moment there's a sunny day, the stains will disappear.

Sorry if this is rambly and confusing, I'm very tired and almost falling asleep as I type. If I make no sense, I'm sure someone will come in to sort you out properly.