I have had very good luck with PhotoPass. The 5x7's are about $12.95 and the 8x10s are in the $18 range. There are discounts for AP holders and maybe DVC folks. I don't know of any others out there.
As far as how many they take, it all depends on the photographer. I have had some photographers take as many as 7 at one time and others only 2 or 3. The nice thing about these photos is that when I get home, they are there on my computer and I can mess around with them. Some I have zoomed in and it has turned into a whole different photo. If I couldn't have done that I would not have bought the photo...just didn't like the distance or what might have been in the shot. So, Disney gets more of my money because of this system. Also, some of the photographers will have the adults stand out of a few shots. This way only the little ones are in the photo and who can resist a shot of just the kids??? Very smart Disney!!