A: With vSphere v4.1, a single cluster can now support up to 32 hosts and 3,000 virtual machines (VMs). That said, always remember that larger clusters present more possible migrations that may be required to restore balance during a DRS pass. Each DRS pass consumes resources and takes time as the service calculates host load and determines which VMs should be migrated.
Many experts recommend keeping cluster sizes somewhere between 16 and 24 hosts to reduce DRS’s workload. This obviously means that environments that go above these maximums will need to spread virtual machines across multiple clusters.