Kubernetes master replicate
Based on official documentation you need to have at least 3 masters in your cluster. Also, you need to have 3 nodes for Etcd cluster, because of quorum. Here is from its documentation:
An etcd cluster needs a majority of nodes, a quorum, to agree on updates to the cluster state. For a cluster with n members, quorum is (n/2)+1.
But, if you have no high load, you can also use your masters for schedule pods there:
kubectl taint node $masternodename node-role.kubernetes.io/master:NoSchedule
Be careful with scheduling of pods which will be running on the master nodes and use limits for them.