You can easily track the past changes in the number of your 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
- Enter a list or segment (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 updating once a day for the All Contacts list and for those lists and segments that have the tracking enabled.