surely it depends on the file size that the camera takes, this is usually selectable in the camera menu (picture size) the smaller and less quality, the less memory needed.
Some camera's will display the amount of pictures that the card will store on each particular setting.
Hi,
On my Fuji Finepix S-7000, I can fit 339 pictures (saved as Jpeg's) at 6.3 Mega-pixels on a 512Mb Xd card. Obviously it depends on the individual camera because of different compression ratios and file formats. A safe estimate would be 250 to 300.