multi site to multi database or multi site to one database?

folks

i recently managed to get the multi-site one codebase to work properly. i am now considering of having a single sign on. however, i note that single sign on will work on a single database instead of multi database. hence i would like to understand how best to configure a single database for multi-sites but single sign on.

also, would like to know the downsides of a single database for multi sites. any views with regards to how many sites can a single database cope with (assuming traffic of 50posts per day for each site).

fazz