The `COUNT` verb is an important feature of the NIPS protocol that allows clients to obtain event counts from connected relays without having to perform expensive queries themselves. This can be useful for many applications such as following user counts, post and reaction counts, etc. The `COUNT` verb accepts a subscription id and filters as specified in the NIP for the `REQ` verb. Multiple filters are OR'd together and aggregated into a single count result. Counts are returned using a `COUNT` response in the form `{"count": }`. Relays may use probabilistic counts to reduce compute requirements.

Link: https://github.com/nostr-protocol/nips/blob/master/45.md