When running up against contact limits, what is the best way to reduce the number of contacts? Currently we plan to disconnect our Sales Cloud and re-sync with more tighter filters on the contacts we need synchronized. We also plan on removing inactive contacts from sendable data extensions. Will these methods lower our contact count or am I missing something?
So first I would identify the contacts that you are currently syncing that you won't be syncing in the future so you can create a DE to use in the contact delete process.
1) Identify the contacts you want to delete
2) Disconnect the Sales Cloud
3) Delete identified contacts and ensure the deletion process completes
4) Re-sync the salescloud with only the contacts that are marketable
For reference to assist with this process Contact Delete Process
Also there is a great help doc to help identify contacts to delete
The SQL query to identify contacts in the sync object but not in all subs which is a great starting point would be
select id from Contact_Salesforce c left join _subscribers s on c.id = s.subscriberkey where s.subscriberkey is null
Also best practice for contact delete with sync data sources:
If you delete contacts in Marketing Cloud and a data source synchronizes before the deletion process completes, the process adds a contact record in All Contacts. This record includes an 8-4-4-4-12 character UUID. To prevent this occurrence, delete contact records from Sales or Service Cloud before deleting contacts in Marketing Cloud. You can also turn off the data synchronization process for that data source. These new records include the actual Salesforce ID in the ID field of the synchronized data source. The _contactkey field contains the UUID-style contact key used in All Contacts.