How many concurrent RDP connections you can run with one XP/VS Server depends on whether you are using a 32Bit or 64Bit operating system.
Windows 7 32Bit has a limitation of max. 3-4GB RAM and only one physical CPU socket
Windows 7 64Bit can address max. 128GB of RAM and also 2 physical CPU sockets
see also :32 bit, 64 bit, operating systems, OS, x64, x86, XP/VS Server