Say Flynn: Is there a limit to the # of plugins per installation you recommend ?

The limit is when your site becomes really slow I'd say.

Or the web server might pass out before that because you exceeded the available PHP memory, which is usually set to 32 MB by most web hosts

A similar question: does the admin slow down when you have a lot of inactive plugins? It would seem that might happen, since you can see the list of all your plugins via the plugins panel.

I guess it's no big deal to just delete the unused ones via FTP. There does not seem to be a way, in WP, to delete from the admin area.

Inactive plugins might be using some PHP memory but shouldn't be using CPU resources. It should not be worth the hassle to uninstall them if you consider using them again in the near future.