If you buy the extensions, they are yours to use on however many projects you like. The only difference between the extension library and actually buying is that with the extension library, you were leasing the extension for your use. With purchasing them, you own the current versions plus any dot releases forever. It's like leasing a car versus buying one... they are both yours to drive - anywhere you want - but you turn in the product at the end of a lease where as now you would own the product.
The solution packs (such as Power CMS and Power Store) have a per license basis.
The extensions, however, can be used on as many projects and sites as you want. They are essentially tools that allow you to build whatever you want - in contrast to solutions - which are completely built when you buy them.
Hope that helps.