Mastering Proxmox - Third Edition

At least three nodes

HA must be configured in a cluster with a minimum of three nodes because with three nodes or more, achieving a quorum is possible. Quorum is the minimum number of votes required for Proxmox cluster operation. This minimum number is the total vote by a majority of the nodes. For example, in a cluster of three Proxmox nodes, a minimum vote of two Proxmox nodes is required to form a quorum. In a cluster with eight nodes, a minimum vote of five Proxmox nodes is required to form a quorum. With just two nodes, the ratio of votes will always be 1:1, so no quorum is possible.