Unable to increase number of vCPUs on virtual machine

Products

• VMware vSphere ESXi

Issue/Introduction

Unable to increase number of vCPUs on an existing virtual machine. 
In the virtual machine's [Edit Settings], less than required numbers available in [CPU] drop down menu. 

Environment

VMware vSphere ESXi

Cause

Maximum number of vCPUs depends on virtual machine hardware versions. 

Resolution

Upgrade the virtual machine hardware version to a newer version that accepts required number of vCPUs. 

Example: An existing virtual machine (hardware version 7) with 8 vCPUs to 16 vCPUs. 
You're unable to increase the number of CPUs from 8 to 16 because hardware version 7 limits maximum number of vCPUs to 8. 
You can upgrade the virtual machine hardware version to 8 (max 32 vCPUs) or later. 

Refer to Hardware Features Available with Virtual Machine Compatibility Settings to identify maximum vCPUs on individual virtual machine hardware version. 

The existing virtual machine's hardware version can be upgraded to the latest version of ESXi in use: Upgrading a virtual machine to the latest hardware version (multiple versions)