The EULA only allows a Mac running Lion or Mountain Lion to run two Virtual Machines running Lion or Mountain Lion.
See page 2 of http://www.apple.com/legal/sla/docs/OSX1082.pdf
Doing so would have no extra cost. It does not appear you are allowed even if you were willing to pay to run 3 or more Mac VMs on the same machine. Therefore if you are trying to have more you would either have to use multiple Macs, or use a different approach.
With Mountain Lion you can have more than two simultaneous users logged in with their own Desktop by using Screen Sharing. Of course they would have to be using a Mac client to exploit this feature although I think ReakVNC Server might allow doing this with non-Macs running the RealVNC client.
Another approach is to run a Mac Terminal server like AquaConnect or iRAPP. I have had a couple of dozen (Windows) clients logged in to a Mac Terminal server this way.
Thank you very much. In my opinion, it would be best if we can utilize 16 GB RAM by using more VMs on the same host. I wish Apple would consider to sell additional vm licenses in the near future.
I don't see it happening. If you run lots of 'Macs' on one physical one Apple loose all those potential sales of Macs.