In theory you only need to identify one discovery host (node) for a new node to be able to join the cluster, but since that node may be down it's better to add several discovery nodes.

In the clusters I work with, where I use the minimum of 3 master eligible nodes, I always identify those 3 nodes in **** but never any of the data nodes, since I will probably never have to add more master eligible nodes while I may continue to add data nodes to expand cluster capacity.

So once you've grown your cluster big enough to warrant a fixed number of master eligible nodes just add those nodes to your discovery settings and never change it again.