There are PhotoPass photographers in all the parks. There is rarely a wait for them. I don't think I've ever had to wait more than 3 or 4 minutes for a PhotoPass shot. There are many of the photographers around, but they are in specific spots, not just roaming around.
You can expect to find PhotoPass photographers near the park entrances, at SOME of the character greeting lines, at some of the classic "photo opportunity" spots, at the Bibbidi Bobbidi Boutique, etc. I would not rely solely on PhotoPass to document your vacation, but you can get some supplements to your photo albums using PhotoPass. For more discussion on this click
here. For information on PhotoPass pricing, click
here.
I'm sure you'll have a great time capturing your memories on film.