For starters, you will be purchasing copies of each edition outright. There is no licensing/cost break on upgrading from the free Express edition.
Next, you are best off contacting Microsoft or some authorized volume liscencing vendor for assistance. Licensing is very confusing. You can purchase number of users, number of connections, per server, per CPU. Even this could be out of date since I checked last. It all depends on how you want to use it.
If you are purchasing Enterprise edition I believe you can install as many instances or virtual machines with a copy as you need as long as they are all on the same server or Hyper-v host.