I have an elastic cluster of 20 nodes, and am planning on setting up a separate monitoring cluster with watcher of 2 nodes. When I request for a license (Gold subscription) for watcher, will it be enough if I get licenses for just the monitoring cluster of 2 nodes?
You need a license for the cluster that is hosting those plugins that require a license: Shield, Watcher, Marvel.
So, in your case you should be fine to have Watcher on that two-node cluster with a license for that cluster.