It would really depend on how demanding each site is.. If they were just plain html page sites I''m sure you could put many, but that would still be limitid to how much bw they use.. I have a VPS-L plan and have 2 forums, and 12 other sites. I don't resell just host what I manage.. My main forum uses around 40gigs of BW
Here are a few of the sites.
Whats your system usage like.(Sytem Resources and Memory?