There is a general guideline for hardware-requirements of the Rancher Server-Nodes itself in the documentation, however, I am not entirely sure if the amount of nodes in the table means per cluster, or in total over all clusters?
Table found here.
Looks like a total number… i.e. for a small deployment it says 5 cluster, 50 nodes. I interpret that to mean 5 clusters of 10 nodes, or 4 3-node HA clusters and a 38 node work cluster.