Recommended is 3 servers, 2 CPU cores, 2 GB of RAM.
However, single server configurations are perfectly acceptable for personal clusters.
I would strongly recommend taking a cluster backup when you get it up and running though, since single cluster configurations do not deal well with unexpected power failures.
Because in larger clusters, the cluster can fix any issues if one hosts goes offline (load balancing) or a database gets corrupted.
In a single cluster configuration, anything going wrong can destroy your cluster's capability. Think of it as RAID, the more drives, the more you can do for high availability.
Running a minimum of 3 servers is just a recommendation, you can run Flynn on a single server (many people do), but then you obviously don't benefit from the HA properties.
However, single server configurations are perfectly acceptable for personal clusters.
I would strongly recommend taking a cluster backup when you get it up and running though, since single cluster configurations do not deal well with unexpected power failures.