I have a 2 GB card, and have about thirty - fourty roms. My wife's DS has a 1 GB card, and I think I put about 16 on hers. So, if I can extrapolate, you will probably fit about 6,7, maybe 8 roms, depending on which ones you choose. Obviously, some are bigger than others. Can't recall the exact size, and am too lazy to look.
By the way, you can pick up a 2 GB Kingston for about 20 - 25 bucks. Worth it, in my opinion.
Hope this helps.
Wiener