Violet - Very hard to say, not sure this will help but here goes...they do go through quite a few when they are little due to sick and leaky nappies. I'd say we used about 2-5 per day when newborn, 2-3 by about 3-6 months.
It all depends on their size for how much "newborn" to buy, DC1 didn't fit in newborn, she was too big (9lb5) so a total waste! She was in 0-3 and "up to 1 month" from birth. Up to 1 month lasted her 3 weeks. My friends baby was 5lb when born, needed "tiny baby" and still in 0-3 when she was 6 months old. I realise this isn't helping. I also found other people bought 3-6 months the most and some 0-3 as gifts.
Last time I bought 12 x newborn, 12 x 0-3, 12 x 3-6 and left the rest as I didn't want all unisex (didn't know the gender and won't this time) and got bought many many more.
Also depends how quickly you can wash them. Our washing machine and dryer can do them in a couple of hours, if you don't have a dryer, you might need more.
Hope that helps!