While it is possible to run Kubernetes on a single machine (for example, using minikube, k3s, or k3d), it is not recommended to do so in production. Single-machine clusters have limited functionality and no failover mechanisms. A typical size for a Kubernetes cluster is six machines or more. Three of the machines then form the control plane. The other three are worker nodes.
The minimum requirement of three machines comes from the fact that this is the minimal number to provide high availability. It is possible to have the control plane nodes also available as worker nodes, although this is not encouraged.