Hi people, Hope I'll got an answer -- is it supported to remove and add servers with mixed roles (control plane and worker)?

It's supported but not recommended, you can set node roles in the cluster.yml:

nodes:
  - address: node01
    user: user1
    role: [worker,controlplane]

Yeah, nobody loves to mess with control panel nodes. I tried with test cluster to remove mixed role node and the cluster become unusable. Documentations is not covering directly such operation.

when you removed the node, how many controlplane nodes were you left with?

After removal, two controlplane nodes.