There is no such rule for Kafka Brokers. Zookeeper should maintain a quorum or (n/2 + 1) total machines (of n) that agree on leader-election values and locks, that results in a total odd number to accommodate for hardware and network failure scenarios. From "Kafka - The Definitive Guide", as well as Apache Zookeeper site, you generally will have negative side effects from having more than 5 or 7 Zookeeper servers total serving applications using it. You should have more than 3 Zookeepers because if one goes down, you are only left with 2, which results in that "split brain". With 5 servers, two can go down, and you still have 2 servers + 1 available for the "tie breaker" vote. For 7, you can loose up to 4 zookeepers and still be good.
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