Hello Simon,

Kafka cluster has nothing to do with number of Orgs and Peers. It depends on your Ordering service and Failover requirement. For e.g.
1. will all Org and Peer use same ordering service and all are on same channel?
2. If you have multiple channels, do want to deploy different Orderer for each/some channel?
3. What is your High Availability and Failover requirement?
Kafka topics ( single-partition topic) are per channel in Fabric.
Typically 4 Kafka and 3 zk. Please read below for  recommended approach and decision
https://hyperledger-fabric.readthedocs.io/en/release-1.1/kafka.html