15 frames on 645 or 12 on 6x6 is perfect for a session. 36 shots on 35mm can be a little much, sometimes it takes me weeks to get through a roll. I still prefer 36 frames per roll over 24 though.
I actually discovered I have a bunch of 10 and 15 shot rolls in the freezer. Expired Agfa HDC and HDC plus. Think I'll break out the F and shoot some oddball stuff. Been wanting to try C41 in Rodinal anyway.