My DS1 was in 0000 for about a month. DS2 only a couple of weeks, he was a big boy.
DD about a month too.
Usually they will need 0000 for the hospital stay at least.
I would get a couple of singlets, or bonds gro suits or whatever you think they will spend the most time in.
Say around 4 singlets, 2-3 gro suits and maybe and outfit or two for the going home out fit. I liked my babies to have a nice going home out fit