Go to "Settings --> Plezi configuration", and choose the "Contacts" tab on the left:
You will find the exact number of your contacts in Plezi & the details of those who are no longer reachable, because their email is no longer active (NPAI) or they have decided to unsubscribe from your communications (Opt-out).
Tips & Tricks
You can delete these NPAI / Opt-Out contacts on this page.
To guide you, here is a short article that summarizes the 2 ways to do it in Plezi.