All depends on how well the server and database beckend are configured.
There is no limit on the domains or virtualhosts a distribution can serve, just on how many concurrent users are visiting a page, and how much load the PHP (or any other interpreted/native server side) applications are asking to the system and database backend.
Since you have a very high amount of RAM, I'd say that you can at least install a PHP opcode cacher and make MySQL cache in memory at least all the index and a few hundreds of the most used tables, depending on their size.
Hire someone to do it.