How many pictures does a 8gb memory card hold with a 10 megapixel camera?

It depends both upon the size of the photos [4" x 6" or A4?] and the quality [Normal? Fine? Superfine?], but an indication can be calculated. My 7MP camera takes:-
- about 2000 4" x 6" photos on a 2GB card,
or
-about 500 A4 photos on a 2GB card,
or
-about 2000 A4 photos on a 8GB card for a 7MP camera,
or
-3000 A4 photos on a 8GB card for a 10MP camera.
However, this only gives an indication: the above figures are for ".jpg" pictures of "Normal" quality in "daylight" taken with a "Canon" camera, but if you are taking "Superfine" pictures at night in ".raw" with another Brand it will be completely different. (Your Instruction Manual should have more details, but if you don't have one they are available on the Net. )