Maximum number of nodes per cluster is 63, max 64 clusters in federation.
Two different technologies can be used to deploy a storage system over multiple sites.
The first is a simple stretched cluster where nodes of one cluster are installed at different sites. This requires persistent reliable low-latency connectivity between all nodes. This is used in small deployments, typically with 3 sites with 3, 6, or 9 nodes.
When a larger storage system is needed a "federated" deployment is used. This is a federation of multiple storage clusters that can operate independently, but with a common namespace. This allows all clusters to operate like one large cluster. This mode allows up to 64 clusters with 63 nodes each. (The supported maximum is 4032 nodes = 64 clusters by 63 nodes). Depending on the specific requirement a single cluster or a federation of multiple clusters can be better.