[Marsymars]

You can do Docker Swarm with a single node (both manager and worker). For a high availability setup, you need a majority of manager nodes for a quorum, so a minimum of three nodes to tolerate a single failure.

I actually just migrated my 4-node homelab from Docker Swarm to standalone instances. My nodes all have very different performance characteristics so I had every one of my services restricted to a specific node - in effect, not making use of most of the useful features of Swarm.

Some features of Swarm are nifty, but in particular I found that a) managing every service onto a single node is counter to the point of Swarm and b) I didn't like any of the options for storage. (1. Local storage, making containers even less portable across nodes. 2. Shared replicated storage, complicated. 3. Online file backend, expensive. 4. NFS shares, and then my NAS is a point of failure for every one of my nodes.)