XP Pro = 2 CPU.
However each CPU socket can have many CORES. So your question may be how many cores. Today we can only get 4 core CPUs and both versions of XP support 4 CORES.
Similar answer at http://www.tomshardware.com/forum/237420-45-question-home-june-30th
What is the maximum number of microprocessors supported by Windows XP.