No. All plans are billed on a minimum number of mailboxes.
The Starter plan minimum is 5 mailboxes
The Pro plan minimum is 5 mailboxes
The Elite plan minimum is 10 mailboxes
The Enterprise plan minimum is 50 mailboxes
This means that the minimum subscription fee for each plan gives you 5, 10 or 50 mailbox licenses to use at your discretion. If you have selected the Pro plan, and only link 1 mailbox to your account, the subscription fee would still be calculated on 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 (on the Starter or Pro plan) after linking the shared mailbox to timetoreply.