All of our stacks are licensed on a per user basis. This means that you (as the purchaser) are licensed to use the stack as much as you want. You can use it on one Mac or multiple Macs, as long as you are the person using them.
If you are a web development agency and employ multiple designers, each developer or designer in your business should have an individual license for a stack. So, if your business employs or contracts 2 developers, you need to purchase a total of 3 licenses for a given stack; 1 for you (assuming you will be using the stacks), and 2 for your developers/designers.
If your business employs a contractor on a temporary basis, and the contractor does not already own the software, you should ensure that the software is removed by the contractor at the end of the engagement. This will ensure that you retain a valid license for this software. By not doing this, transferring the software to the contractor, transfers the ownership to them. You would need to purchase a new license if you intended to use the software again.
Total CMS is licensed differently. The stacks themselves are free. However, to use the CMS itself is licensed to a domain, not to a user. For more information about Total CMS licenses, see these FAQs.