Depends on the size of the game: GBA games vary between 4MB, 8MB and 16MB. (32MB for Kingdom Hearts but that one's just weird)
256mb = 32MB.
So just do the math
I get about 5-6 games on mine on average, but it really depends what you want to play. And you can trim the games down a bit to fit more in, but I've never bothered with that so don't know if it makes much difference.
[Edit] As for recommending it depends on your playing style. I tend to work through games from start to finish, so 5's plenty for me (2 platformers, 2 puzzle and whatever will fit in the remaining space usually). That's not to say I can only play 5 games at a time; if I switch to a different game I transfer the save to my hard drive and swap the cart contents, but I dont do this often. Usually once a month.
But if you're someone who wants instant access to your whole collection or changes through more than 5 games every day you might want to look for something bigger.
KH is the only one I know of that'll take up the whole cart in one go
FF:TA and some of the newer platformers manage to hit the 16MB mark, you just have to carefully pick and choose which games you want to play simultaniously to fit more in.
PharaohsVizier's right, the 256 carts are *very* outdated now (got mine 3 years ago) and you could probably get a different cart with more storage space for the same amount of money if you look around.