Please note that contacts are available only on Marketing plans.
You can easily track previous changes to the number of contacts in the Contact History tab. It is located on the Contacts page of your dashboard:
The default view on this page shows the history of your All Contacts list. However, you can add up to 10 more lists to view their History:
Go to Contacts
Click the three-dot menu besides a list or segment of your choosing (all except All Contacts which already is on the Contact History.)
Select Edit List (Or Edit Segment if segment was selected)
On the resulting popup, select "Track list history/Track segment history and click Save
Afterwards this list or segment will be visible in Contact History:
The Contact history list is updated once a day for the All Contacts list and for those lists and segments that have the tracking enabled.