Currently, the recommended limit of Kudu masters in multi-master deployment is 3 (i.e. no more than 3 masters is recommended): https://github.com/apache/kudu/blob/branch-1.9.x/docs/known_issues.adoc#scale
It would be nice to clarify whether there is anything substantial behind that limit. As of now the recommendation stems from the fact that all of our multi-master tests and tested deployments use 3 masters. Overall, being able to deploy 5 or more masters in case of bigger clusters makes sense from the HA perspective.