From the recent exercise of unfolding Salesforce comercial contracts for Marketing Cloud, we spoke about the concept of Supermessages and how to streamline the use. https://salesforceweek.ly/2020/03/get-the-most-out-of-your-super-messages.html
In doing so, it also unfolded how Contacts works and are counted. And with it some of the things you may want to bear in mind in Marketing Cloud to get the most of your Contact contract allowance.
What are you syncing?
You can quickly run a query that looks at the first 3 characters of the SubscriberKey, there you can see if there are Salesforce records.
From there you may see records starting with 005, that would mean you have the object User synced. There you have an example of easy saving, disconnect objects you don’t use and remove them. As otherwise they are just eating up your allowance.
You can also ask nicely to your Account Executive for a visual report with a breakdown of Contacts usage in Marketing Cloud, similar the below and to the one we covered about SuperMessages.

Test subscribers
Could it be that you have a large number of ‘test’ Subscribers where their SubscriberKey is their EmailAddress, or something other than the Salesforce ID you generally use.
Triggered Sends
Have a look at Triggered Sends Managed List, then check if these SubscriberKeys are present on the All Subscribers List in Email Studio, or All Contacts List in Contact Builder. If they are, happy news.
If not it means the Triggered Sends are not automatically adding Subscribers to the All Subscribers List, so that these records are visible to use, segment on or even delete if you wish.
MobileConnect
This one is more of an FYI than a streamline opportunity.
The All Subscribers list does not comprise of all your ‘Contacts’. Yep, may sound weird but each contact is one channel. So for example, a mobile connect contact is one count of a contact and you will have another one for their email in Email Studio.

And thats a wrap, for now, I hope you find these items useful to have a good eye and get the most out of your Marketing Cloud contract!