It depends.
If your cameras sensor is 12 mp and you are shooting in RAW+JPEG Fine, a 8 gb card will just about hold 225 different images. That means you would have to use two 8 gb cards or one 16. Actually I find using 4 gb cards more effient, since it takes less time to copy a hundred images per card than three hundred or so.
Last night I shot at a drag-strip and used 2 gb cards .. after 52 shots, I just changed cards. Easy
And of course there is the "all your eggs in one basket" rule
I would have guessed that you could just put the card you have in the camera and then do the math
