Hmm... I would start with 6 of each for a newborn, to see how sicky he is, you may not need to change that often - Jasper did, but he throws up after EACH and EVERY feed but I had dozens given to me as gifts though! I also would get 6 newborn size and then more in the next size up, because if they're more for 'back up' then it doesn't matter if they're a bit loose, and that way you'll get more wear out of them.
For Jasper now,who's 4 months, I have 8 full body suits and about 8 vests - plus 3 pairs of pants, maybe 8 short and long sleep t-shirts, and mayb 5 pairs of socks. This gets me through if I do washing about every other day - sometimes more or less, depending on how much he throws up. We have a dryer, which I do use, so my washing is through and back ready to wear in about 3 hours.