1. You have to shutdown the VM to edit the number of vCPUs (unless you enabled CPU hotadd), from your screenshot it appears the VM is still running thus preventing you from editing that option.
2. Make sure the VM's virtual hardware version is at least v8, since v7 only allows up to 8 vCPUs.
3. Your physical ESXi host should have at least 16 CPU threads as well.
4. You need at least vSphere Enterprise licensing to assign more than 8 vCPU to a VM.
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