Symantec EV 10 licensing change from EV 9 ?

Hi People,

Can anyone here please explains to me what is the proper way to count the license required for EV 10 ?

My understanding is that it uses the total active mailbox on my Exchange Server with the active AD user account determines the total amount of EV 10 Exchange license needed, not the previously archived mailbox in EV.

here's the example for my case:

Company size: 850 user
Exchange Mailbox total count: 1257 (including shared mailboxes and meeting rooms)
EV9 archived mailbox count: 2933 mailbox vaults.

so do I just need to get 850 licenses of EV v10 to cover the usage ?