I purchased a license and would like to know how many domains I can use it on.
In my case, I would use it in the development of an internal system, within the network, and an external system for my clients.
Yes, you can do so. Use it for your development and for your client. In case if you reuse your solution for another client you will need to purchase a new license for your new client. Each license allows single deployment of the end product/site.