It is hard to say since each brand runs different. LO is almost 5 months and fits in anything from 0-3 months to 9-12 (gerber runs really small).
I would say only have a couple of newborn, and then a variety of 0-3 and 3-6. you can always return some of the clothes and keep the store creadit and buy what you need as you need it.
IMO it i better to have items bigger, since he/she can grow into them, but you will want some items that fit great...also I would recomend you don't wash everything right now...so that you can return/exchange items if LO grown out of one size quickly...I wish someone had told me that before I rushed and washed everything...DS only got to wear things once or twice since I had too much in one size...