With XSitePro you get one license, but you can install this on up to two computers at a time.
If you update your computer, you can un-install XSitePro and re-install it on the new one.
There is no limit on the number of times you can do this (several times a day if you wish :) ).

You can upgrade your licenses to allow additional PC's to run XSitePro.
Each upgrade will add one more install available to run XSitePro.
So to run 4 computers at the same time you will need two of these upgrades at $97 each from this link:
http://www.xsitepro.com/extra_license.html

Find out more about XSitePro website design software at:
http://www.xsitepro.com

And explore the new features of XSitePro version 2.5 at:
http://www.xsitepro.com/new-version-features.html