rule to set number of brokers for each server

Dear all

I am provision production kafka cluster, which has 3 servers, I am
wondering how many brokers I should set for each servers, I set 3 brokers
in dev clusters, but I really don't what is the advantages to set more than
1 broker for each server, what about 1 broker for each server, totally 3
brokers, instead of 9 brokers.

thanks

Alec Li