One or more virtual processors can be defined in the VM, but the amount cannot exceed the number of logical processors (sockets x cores x 2 if hyperthreading is enabled) present in the host. The number of vCPU sockets specified in the configuration determines the number of cores available. One VM could have virtual sockets and virtual cores. By default, for each vCPU, a single socket with a single core is assigned as virtual hardware. You can change the default behavior and assign multiple CPU cores per single socket. You might need to do this for licensing reasons, for applications running inside the guest OS (for example, the latest SQL Express can work with more cores, but not with more sockets).

If you have more than eight vCPUs, virtual NUMA (vNUMA) is enabled, and ESXi distributes the VMs in more NUMA nodes if it is not possible to fit them in just 1.

If you are running a vSphere Enterprise Plus license, you can also enable Hot Plug for CPU. This option will enable you to add more vCPUs even if the virtual machine is running. You are usually not allowed to change the number of vCPUs while the virtual machine is powered on.

Hot Plug for CPU can be easily enabled by selecting the Enable CPU Hot Add checkbox:

The maximum number of supported vCPUs per VM is 128.