Please bear in mind that the second thread is a achieved by Hyperthreading so is not equivalent to a true core.
Without knowing other information like memory network capacity, processor usage etc. and basing it on purely vCPU then 8 would be minimum but would give little room for moving machines around for patching ESXi, covering off high CPU workloads or providing resilience for potential hardware failures, I would recommend a minimum n+1 so 9 hosts with 2 processors (32cores) = 18 VMware processor licences, if I remember correctly you buy these in 2 processor bundles but always check with your VMware reseller.
