I have just deleted some contacts we didn't need/want in the "database" but the overall total is still showing the original quantity? When will this change?
If you deleted the contacts from SFDC, then they will remain in Marketo until they are deleted there. I would create a smart list that has the filter "SFDC Is Deleted: True" to isolate them for deletion. Additionally, the "All People" system smart list will update quickly.