you can have as many as you want. if you go to start> Windows Virtual PC, there will be a button at the top of that window called "Create Virtual Machine". Pretty sure you can use this to make as many as you want.
However, it seems as though these VM's cannot be backed up. That is, I tried backing mine up because I had to reformat my computer, and I was not able to run them again properly with the new OS, so it seems as though they are somehow bound to the OS they were made in. For this reason, most people might opt to use vmware.
It is very good for legacy apps. I have to use it at work in order to use Cisco's VPN Client app on my 64 bit workstation. I now have an entry for it that runs under 32 bit Windows XP as if it were any other application on my system. I also added the copy of Remote Desktop from there to my start menu.