How many photos will 8gb hold on a 18mp camera?

Kinda depends, generally in my experience the size of a jpeg high res image directly from, say a canon t2i, is between 5.5 and 11mb, some as much as 16mb. An estimate therefore would be approximately 2979, 1489, and 1024 pics respectively. This however is only an estimate as each picture has different information which takes up more or less space than the previous picture even on the same device. Aditionally, it's unlikely that you would be using or keeping pictures in raw, but if you are, expect considerably less than that, on average canon raw files are around twice the size and generally save jpeg as well for menu images. So it would put that average number at around 750 or so pics, but most likely less.