Hi,

It depends on the nomber of mailbox to be archived by enterprise vault.

Agreed. It is by the number active mailboxes being archived. You dont need a license for Mailbox Archiving for people who have left the company just do a final archive and close the partition before you delete the user account

I think it's actually more like the number of "people" who use the mailboxes. So group mailboxes and things do not really count.

But it's hard to get a definitive number here ;)

Per the licensing guide

PRODUCT OFFERING                                              METER LICENSING                                                                       DEFINITION
Storage Management                                                         Per User                                                      Total number of current (active) Users to be archived (typically all Standard/Advanced/Enterprise                                                                                                               employees in company). We do not require a license for inactive
Editions                                                                                                                                                       mailboxes, system/group mailboxes or Users with multiple mailboxes

                                                                          
So basically all 3 posters are correct.   There is no link to the archive or use of it as far as the licensing goes.   You could assign rights to the archive to another enabled user and add more content if you wanted without requiring a license.