How to license Azure stack?

Azure stack can be licensed using EA or CSP licensing channels. Partner also can leverage SPLA licensing model for Azure stack. For more details around licensing options refer to the document here

Azure stack is deployed in either connected mode or in disconnected mode. Connected mode allows Azure stack to send consumption data to Azure metering engine. In scenarios where Azure stack cannot connect to the internet, it must be deployed in disconnected mode. The licensing option suitable for disconnected mode is EA only, while EA or CSP licensing options can be used in connected mode.

Choosing a billing model guidance here