We will have 15 CCR clusters eventually.
As you will not be archiving the edge and cas/hub server (as they host no mailboxes), you do not need a EV systemmailbox there.
As far as CCR goes:
You have physical node A and B (active and passive)
You have 1 Exchange server (let's call that C) running on A. If A fails, C will failover to B, and due to the log-shipping, continue to work.
Hence, although you have 2 nodes, you will only have 1 exchange-server. Hence, you only need 1 EV-systemmailbox.
In EV you target the ExchangeServer to archive, not the nodes. As you have only 1 exchangeserver, EV only archives from 1 exchangeserver. As far as EV is concerned, if C runs on A or on B, does not matter.
Hope this is clear.