Yes, the limit recommendation is based on number of nodes and not clusters.
The recommended limit per UM instance is 48 nodes, which means that user can monitor either 24 2-node clusters or 12 4-node clusters, or 6 8-node clusters, or mixture of these but total count should be 48 nodes.
However, the product doesnt have any restriction and it'll allow you to add more than 48 nodes. But, the recommended number is 48 nodes.
Regards,
Saravanan