I had the same situation before and luckily my friend recovered the images that were lost using a software. The main rule is not to shoot anything on the card after its been formatted. If you overwrite it with a new data, you might not be able to recover previously lost images.
go to the sandisk website...they have software that can recover data, they say even if you have erased the pictures. I have never used the software, never found the need. I usually loose the disk before I wear them out.
The following website by SanDisk answers your question, considering the megapixels and file size: http://kb.sandisk.com/app/answers/detail/a_id/69/~/number-of-pictures-that-can-be-stored-on-a-memory-device