I'd have to say that Jennifer W's list is spot-on for what I'd go for my son. I figure I do laundry once or twice a week for my kids' stuff, so I want to have enough on hand that it's not an issue if I don't get to it, and so that I can add some variety. I figure that if I can clothe them for two weeks before I run out of clothes entirely - then I don't need more.
for my son:
14 pairs underwear
14 pairs socks (about 10 sport/4 "nice")
10-12 mixed t-shirts/tanks for summer
2 craft/paint shirt & short outfits or 1 decent smock
10-12 pairs shorts
2-4 nice shirts for family gatherings
2-4 nice pants/shorts for family gatherings
1 pair loafers
1 pair sneakers
1 pair sandals
1 pair aqua shoes or a second pair of sandals that will hold up for wet wear
1-2 swim suit/trunks(I got lucky and found two this year at a resale event and we have a sandy beach by us so I snagged 'em)
1 sun coverup - this can just be an extra t-shirt for a boy
1 light jacket or sweater for chilly nights
1 sunhat (still need to find this one)
1 raincoat & rainboots
I think that covers it. For my daughter, I'd do approximately the same, just sub in some dresses for some of the shirts, add some cute dressier shoes instead of loafers, and have some of the shorts be lightweight cotton ones that she can wear under the dresses, especially during play.
If you are in the habit of doing laundry every day, you might be able to cut some of those numbers down in half if you like. I've tried, but I've got too much going on to do laundry that often. :-)
And I LOVE the resales & hand-me-downs!