Reaction events can be anywhere. Everywhere. The only way to count for sure is to look EVERYWHERE. That is a problem when there are a thousand relays.

Relays could signal support for having some back-end broad counting system, and the COUNT command, and then you could just cheat by copying the answer off of a relay that you trust who has already asked thousands of relays, so that your client won't need to. I don't know how well this idea has been implemented on relay-side or client-side. Gossip client is not doing it.