If you have a wider family and friends you'll get bought absolutely loads! I'm having twins and have ordered a pack of 3 sleepsuits for each baby (boy and girl) in tiny baby size and also in newborn size. (As not sure how big they'll be when they come out) Same quantity again and sizes in vests.
With older child we got bought tonnes of 0-3 sizes but didn't have a lot of newborn (which they won't wear for very long anyway) so I'd say 7 days worth of vests and sleepsuits
Also where the clothes come from differed hugely - Sainsbury Tu range and mamas and papas come up huge so sized down in them whereas next and boots tend to be a bit more true to size
I'd buy just a couple of things initially and wait to see what size the baby is when it comes out - that way you don't buy loads of really small baby clothes that won't fit