They'll send you as many CD's as it takes to get your pics to you. I read on here where someone had 3 CD's sent to them to get all their pics. I just got mine back and it has 286 pics on one CD.
I would imagine they are the same as a regular CD and can hold 700MB worth of data, as usually all pictures are different sizes, it'll depend on the size of the files for how many pics will fit on one CD.