But aren't there huge lists of all discovered relays available? However I agree with you that the measurement proposed is "minimum count".
Discussion
"Discovered" being the key word. And just because a relay is discovered, doesn't mean you have access to its events. And even if you had access to the events, that doesn't mean you know how many people are using it. You only know which events are stored in it, and only If they haven't been deleted.