ooooohhhhh... i get what you mean now.
- i have one older site that i touch maybe once a month
- i have one main site that i work on daily
- i have two new sites coming online in the next weeks that will get their content from the same db as my main site
in my opinion, the way that one can run multiple sites at a time is to have them all talk to the same database and/or content management system. this way, you make one update and it is reflected everywhere.
i am sure that there is much more to it than that, and i hope that some of the more experienced folks post a reply with more details / tips for running a large number of sites.