How does licensing work for a Splunk Heavy Forwarder and Indexer?

Hi, Need a little insight on how licensing for a Heavy forwarder works: We are planning a solution for Client where we might have one instance of Heavy forwarders and two instances of indexers per environment with one Splunk Enterprise instance. And as per my understanding, we can use the Splunk Enterprise instance as heavy forwarder. Now the question is to which instance do we need to procure the license and need to apply it? And how can indexers be configured to have one license instead of multiple instances? Thanks, Ram