If you dump to the /dev/null file, the output goes nowhere (which is faster than storing in a file), you don’t get the side effect of a really big file, and you still get the number of messages reported to you.
So hcidump -r -s 7 target_connection /dev/null should report the number of messages
You may be able to do the same thing on Windows by specifying the output file as NUL:, but I have no way of testing it.
