In the Dynamics 365 for Marketing online application you are supposed to be able to see the number of "Marketing Contacts" allocated and used. This category doesn't appear as one of the Quota names. I can see "Marketing email messages", "Total contacts in database" and "Litmus inbox previews" but not "Marketing contacts" as this document states: https://docs.microsoft.com/en-us/dynamics365/customer-engagement/marketing/quota-management
I have run a few Customer Journeys for test purposes and would think I could see the contacts that were sent emails in the Used list.