I don't mind one at a time, but I have probably reloaded XP about 6 times since I've had it. I build my own PC's and like to tinker and change things around a bit.
If you re-install it on multiple computers or a single computer 25 times, you'll be asked to get a new 52-digit number from Microsoft. The worst part of that is, even if you upgrade say a hard drive, install a new hard drive, install a new piece of hardware like a sound card or video card, those count against you too, and you will eventually have to "reauthorize" an already legitimate and genuine installation of Windows XP.
Acquiring the 52-digit number from MS is free, but it's a pain in the a$$.