As others said, you are clear here. Licensing is by the number of servers deployed, and the number of users on the network. AD is covered by those things, it's an "included feature" once you are covered otherwise. There is no specific licensing for AD, only for the server/user combo. So you can deploy AD in any configuration that makes sense for you. The licensing will remain the same whether you use AD in some complicated matter, just one domain, or even not at all.