API call to return number of contacts?

Summary:

Is there an API call that can be made that will return the number of contacts in the instance? I looked through this document and didn't find anything:

We're an enterprise customer with multiple instances and we have a certain number of contacts we're allowed in our contract across all instances. I'm trying to find a way to build a report that can show the counts across all instances in one place instead of us having to log into each instance manually to get these counts.

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