Generally you should get around 660 photos on an 8gb card. The way I calculated this is that 8gb is around 8000mb divide that by 12.1mb and you get around 660.
Remember though, that if you are using the card in a camera/phone which allows for different sizes and quality, this will greatly affect the amount of photos that will fit onto the memory card. You will get many low resolution photos but much less with larger, better quality ones.
If you have another (bigger or smaller storage size) card in the device at the moment, you should be able to work out an approximate number based on how much bigger (or smaller) the new card is.
