There are many factors that will affect the number like what are the VMs doing - how much memory does your ESX hosts have? how many Nics? what type of sotrage? how many cpus/cores? - for you can have 4-8 virtual cpus per core - so a quad - quad core host theoretically could have 64-128 single vcpus VMs - but if it only has 8 GB of RAM you will not get that many -
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To give some possible values:
each core can run 4-6 vCPU (old CPU like your maybe less and depends also by VM CPU load)
increase the RAM by a 1.5 factor, so with 10 GB RAM you probably can overcommit to 15 GB vRAM (this factor could vary a lot)
on each group of physical disks consider 10-15 max VMs (vary on VM I/O operations), less with SATA disks
Than take the smalles number that you got from those rules.