The license/subscription for use of Extensions is based on the total number of users in your Tableau environment (so the Total of all Tableau Creator, Explorer and Viewer licenses). If you are on a Tableau Core model we use the average of 1000 users per 8-core license. So an Extension Subscription is based on the size of your Tableau implementation. If you are on an active subscription you can also use the extension in a DEV or TEST environment without counting your test and dev users as extension users.