If we only want to measure 1 shared mailbox, do we only pay for 1 license?

All plans are billed on a minimum of 5 mailboxes.

If a customer sends an email to your shared mailbox, and you have team members replying to that customer from their own email addresses, then you’d link both the shared mailbox and the individual team member mailboxes. This allows timetoreply to track the performance of both the shared mailbox and the team members who are replying to the emails.

If the team members are simply replying from the shared mailbox address itself, then you would not need to add the team members’ individual mailboxes, but it would make sense to do so as you’d have 4 available licenses left after linking the shared mailbox to timetoreply.