One relay is going to report 5, another one 7. How do you combine them? Max? Sum? Guess?
But, push a new meta event from the relay. If clients want to use it, that's great. If not, that's also great.
Discussion
Use the highest count
The relays might have completely different people pushing to them though, and then you'd need to sum them.
If it happened a lot you could include some optimized data hinting at the npubs behind the counts. It's not an easy problem.
Smart relays using negentropy to stay in sync with others in the network. You could even expose count over HTTP given an event ID.
It's better to keep information in the messages themselves so the protocol stays focused. But if relays want to talk to each other using some sort of magic that's cool too. At the end of the day, most clients will be using more than one relay, so they need to combine that information somehow.