Easiest thing to do is use a different database prefix for each site.
When you install CMS through the installer, there is an option to change the table prefix. It defaults to cms_, but you can use whatever you want. This allows multiple sites to coexist.
However, if you're going to be running more than 5-10 sites, I would recommend setting up multiple databases, since I'm sure this would cause a strain. Don't take that as concrete, though. I'm not a mysql expert by any means...
It probably would be worth experimenting a little. Or at least looking into some information on mysql performance.
We do a small amount of hosting here at work. It's not our main business, so we're not experts. But, we give a different db for each domain mainly for security sake, but we also have a serious hunch that it will help performance at a sacrifice for disk space.
Hope that helps.
