Number of hosts that are part of a VMware Clustering depends on your vSphere version maximum capabilities, for example in vSphere 6.7 it's 64 host nodes per each cluster.

But to calculate required host you need to estimate current load + required failover load + Fault Tolerance Load, based on your needs join required ESXi hosts to the cluster and then based on previously mentioned factors, now you can configure HA Admission Control: Specify exact hosts for failover or reserve sufficient physical resources (CPU, RAM)