IIRC this feature does only take the pCPU (not the HT ones) into account. Also there is one Core reserved for the System. So in your example 6 pCPUs - 1 pCPU Reserve is a max. of 5 vCPU VM if you would like to use this features. NUMA also comes into the game but you have only a 1 socket box so we dont care right now.
You have only a very small Host and its way overprovisioned with a 8 vCPU VM.
Regards,
Joerg