Thanks for reaching out!
You will need to purchase license only for the developers that will use Telerik JustMock in design time.
The licensing for our .NET products is developer based. This means that each developer using our products in design time needs a developer seat/license. Our products do not work with license keys and you can safely install them on any machine without worrying about license key errors. In this way the other team members such as designers, QAs, etc. can also work on the solution. In other words, we do not impose any limitations on the number of projects, machines or servers the products are used on, as long as they are used by the same licensed developers.
More details are provided here: http://blogs.telerik.com/VassilTerziev/Posts/08-01-15/Telerik_licensing_demystified.aspx?ReturnURL=%2fVassilTerziev%2fPosts.aspx
In case of any further queries, I remain at your disposal!
the Telerik team