Mint.HTTP1.open_request_count

You're seeing just the function open_request_count, go back to Mint.HTTP1 module for more information.
Link to this function

open_request_count(conn)

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Specs

open_request_count(t()) :: non_neg_integer()

See Mint.HTTP.open_request_count/1.

In HTTP/1, the number of open requests is the number of pipelined requests.