It sounds like you have already sized your cluster and have it up and running. How is it performing?
You generally should have at least 3 master-eligible nodes, but 3 is often the optimal number. If your cluster is deployed across a larger number of availability zones/racks you may increase this.
This depends a lot on the use case, as coordinating-only nodes are often not required. If you have a lot of querying you can add this type of node to see if it improves performance. Set the number so they are under reasonable load.