Site Sculptor has a developer license like all of the other extensions, so you can build as many sites as you want with it. The CMS in Site Sculptor is not the same as PowerCMS 2. It is similar and offers quite a bit, but PowerCMS 2 has some more features.
From the FAQ:
The built-in CMS included with Site Sculptor is a simplified version of PowerCMS, providing you with essential capabilities as it relates to updating content on your website. In addition, Site Sculptor’s content management feature includes the ability to specify Page Title and meta keywords. If you wish to utilize the full fledged version of PowerCMS with Site Sculptor you can do so by overwriting the CMS files generated by Site Sculptor. 
-justin


