Unfortunately, there is not currently an API endpoint for getting license usage.
Depending on what you are trying to do, there are a couple options. You can export the User Records (Administration | People | More Actions | Export) and from there you can see how many people there are in each role. You can also see when each person last logged into the system. So if there are people who have licenses and have never logged in, you can deactivate their account. Same for people who are no longer at your company.
Alternatively, if you have Enterprise Insights, you can look at the user information there and do similar queries.
A third option might be to use the Users API and get a list of users; that includes the Role ID, which you could then map in the same way as above with the export file.
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