How many is too many in your galleries

Is there an upper limit on how many images to include on your website?  Right now I have each of my 3 galleries broken up in to pages with 15 thumbnails, but as I add images they're tacked on to the front so my most recent stuff is always at the start.  How many pages do you think would be a good max?  I've taken a few images off that I don't like anymore, but tend to just keep tacking on.

And along the same lines, do y'all prefer thumbnail galleries or just scrolling through a series of full sized images?  I guess more importantly, which would potential clients prefer?  I'm considering ditching the thumbnail pages and just going straight to a 600px image with Next & Previous links.