If you search with an asterisk “*” in Stackoverflow, the number of results you will see is equal to the total count of questions plus the total count of answers to any question.
Glean consolidates the question and any answers posted on it as a single document. Because of this, the document count you see in the admin console in Glean could be less than the number you see in StackOverflow.
To verify the document count, use the filter in the Stackoverflow search for questions as `is:question`.
The number of results you will see in the output should match with the number of indexed documents count in the Glean admin console.