I think this might be smoother as

> A client subscribes to a `topic` on a `channel` over a TCP connection per `nsqd` instance.

or maybe

> A client subscribes to a `topic` on a `channel` over TCP connections to one or more `nsqd` instances.

I think I might understand why this is tricky to word: It's one TCP connection per client per `nsqd` instance, but there might be more than one `nsqd` instance to connect to.  Feel free to leave this as-is.