How many pictures can a 8gb memory card hold on a 12 mega pixl camera?

i have a 12 mega pixl digital slr camera (canon eos 450 d) and i dont yet have a memory card, i am not sure what brand of memory card to buy and what brand will hold the most pictures in the best quality

What setting are you using?

It could be anywhere from 1500 to 5000+. It all depends on the picture quality.

Your Instruction Manual has all the answers you need. Just turn to page 64. It will show you that (for every 2GB) you will be able to shoot approx...
460 high quality jpegs (at approx. 4.3MB each) or
120 RAW images (at 15.3MB each) or
99 RAW + Jpeg (at 15.3MB & 4.3MB each)

So, if buying an 8GB card, just multiply these figures by 4.

For the 450D, one of the best cards to buy would be the high speed (and very reliable & popular), Sandisk Extreme III SDHC card (which also comes with an SDHC USB 2.0 card reader).

Make sure you Format it before you use it the first time, and also everytime AFTER you have uploaded all images to your computer and you then wish to delete all images from the card.

Worst case 667 photos if every shot was12 MB. However, the camera will be able to reduce the file size to something smaller depending on the conditions and content of the photo. You could expect to get 750 easily at the highest resolution. If you choose a lower resolution, your capacity will multiply.

In the manual, Canon usually lists how many photos a 512 MB card will hold. Since two 512 cards makes a gig, multiply the number by 16 and you'll get an approximate number.

Keep in mind that there are JPG / RAW / JPG & RAW formats, too.