"Only on the machine" ?? You referenced going into the registry or Group Policy Editor, so I assumed it was the Windows logon password you want to be numbers only and my instructions will do that.
If the Group policy is setup to require password complexity, then a set of rules for what the password consist of is required. If that policy is disabled a password can be as loose as just the number 2 or a single letter, whatever you want.
I have a machine that is not connected to Network or any domain controller.
I need to enfore that any user creating aWin account shoud use only Numerics digits as Password on the machine. I checked Strong Password Policy but we can set enable/disable using "AlphanumPwds" as 0 or 1. To ensure that user must not be able to enter Alphabets characters, how can we enforce? User should be able to enter Numerics only as Password. It could be min 8 and max. 15 digits.