WordPress themes here at Shaped Pixels, both commercially priced and free, are licensed under the GNU general public license. This means that my themes may be used on as many websites as you like and have no restrictions.
However, I would recommend you read the GPL Licensing so you get a better understanding how this type of license works.
Domain Licenses. Although our themes are GPL Licensed as noted above, Shaped Pixels now assigns “Domain License Keys” for our theme and plugins. This does not restrict your usage of our themes and are used only for services we offer for automatic theme updates and support. Automatic updates are available for our themes and plugins obtained from ShapedPixels.com and are available as long as the license key generated from the purchase is valid and active.