The current vSphere (HA/DRS) cluster limit is 32 ESXi hosts for vSphere 5.5 and 64 for vSphere 6. Scalability of additional components (storage LUNs, network ports, and so on) might impact cluster sizes. The minimum cluster size is 2 ESXi hosts (with N+1 redundancy). Depending on the procurement cycle for new hardware, a number of standby ESXi hosts might be kept ready to add to any given cluster.