When you select a custom configuration, the New Virtual Machine wizard prompts you to specify the number of processors for the virtual machine.

Specifying multiple virtual processors is supported only on host machines that have at least two logical processors. Single-processor hosts that have hyperthreading enabled or dual-core CPUs are considered to have two logical processors. Multiprocessor hosts that have two CPUs are considered to have at least two logical processors, regardless of whether they are dual-core or have hyperthreading enabled.

For Windows virtual machines running mostly office and Internet productivity applications, using multiple virtual processors is not beneficial, so the default single virtual processor is ideal. For server workloads and data-intensive computing applications, adding extra virtual processors may provide an increase in application performance.
Application Recommended number of processors
Desktop applications 1 processor
Server operating systems 2 processors
Video encoding, modeling, and scientific 4 processors

In some circumstances, adding additional processors can decrease the overall performance of the virtual machine and your computer. This can occur if the operating system or application is not using the processors efficiently. In this case, reducing the number of processors is recommended.

Assigning all processors on your computer to the virtual machine results in extremely poor performance. The host operating system must continue to perform background tasks even if no applications are running. If you assign all processors to a virtual machine, this prevents important tasks from being completed.

For more information about virtual processors, see Virtual Machine Processor Support.