I am currently running over 13000 nodes with a 5 server cluster. These are large boxes (90gb memory, 64 cores) but cpu usage is under 20% and memory usage is under 30%.
We run a Raft cluster of 5 voting members with 13,000 physical Nomad nodes mixed across OS's with a bunch of random workloads using docker, exec, raw_exec, java, and some in-house drivers. I'll clarify that (and I can't... because I can't edit my post anymore :( ).
We're nowhere near that size, but the architecture is probably identical.