You should observe certain guidelines when configuring Fault Tolerance.
In addition to non-fault tolerant virtual machines, you should have no more than four fault tolerant virtual machines (primaries or secondaries) on any single host. The number of fault tolerant virtual machines that you can safely run on each host is based on the sizes and workloads of the ESXi host and virtual machines, all of which can vary.
If you are using NFS to access shared storage, use dedicated NAS hardware with at least a 1Gbit NIC to obtain the network performance required for Fault Tolerance to work properly.
Ensure that a resource pool containing fault tolerant virtual machines has excess memory above the memory size of the virtual machines. The memory reservation of a fault tolerant virtual machine is set to the virtual machine's memory size when Fault Tolerance is turned on. Without this excess in the resource pool, there might not be any memory available to use as overhead memory.
Use a maximum of 16 virtual disks per fault tolerant virtual machine.
To ensure redundancy and maximum Fault Tolerance protection, you should have a minimum of three hosts in the cluster. In a failover situation, this provides a host that can accommodate the new Secondary VM that is created.