How many clusters you need per number of clients

how many nomad clients can one nomad cluster (of 3 servers) support?

"Nomad has been proven to scale to cluster sizes that exceed 10,000 nodes in real-world production environments. "

also note "The recommended deployment is either 3 or 5 servers. "

As @resmo said, we’ve seen people running successfully with over 10K nodes in a single cluster. That said it really depends on what kind of hardware you have for your server nodes, and how many jobs you have running on your cluster. It is unfortunately kind of tough to give exact prescriptive guidance since there are a lot of variables at play.