Fifty photos is a reasonable figure for each gallery – you can have more but the gallery will eventually run more slowly.
There are no fixed limits to the number of galleries in svManager. Most of the heavy work is done on the server and I can't speak for every server out there but most servers should cope with 100 galleries easily.
You are right to be thinking about the issues of organising your galleries. There are no sub-categories in the svManager gallery listing but you can sort them by gallery title or date. You can also have multiple svManager installations on the same server but that can become tedious in itself, especially when you want to upgrade to a new version of svManager.
AutoViewer is on the 'To Do' list but I can't give any time estimates yet.
SimpleViewer Support Team.