Why is there a strict rule of number of brokers as multiple of AZ?

We keep a consistent number of brokers across AZs and recommend that replication factors are set to at least 3 and replicas placed to ensure placement across all 3 AZs. This is intended to preserve the durability and availability of your cluster in case of a failure in an AZ. The availability aspect is the most significant factor in balancing brokers across AZs, since the loss of a single AZ in an unbalanced cluster would have a disproportional impact on the other two AZ's brokers, and lead to unpredictable availability. This explains why this is a best practice. The service design enforces this best practice, while simplifying operations, by keeping the broker numbers synchronized across AZs.