MY DESK TOP MAC IS ASKING FOR A PIN CODE WHEN i LOG IN HOW DO I FIND THAT?

Have you inadvertently locked your Mac with Find My?

Please see the section of the following Apple document that describes the startup screen that requests a PIN.

If your Mac doesn't start up all the way - Apple Support

Then, please view the following (also linked in the article above):

If your Mac is lost or stolen - Apple Support

Please read Writing an effective Apple Support Communities Question and then re-write your question with pertinent infomation. Mac Desktops have been made since 1984 in thousands of configurations. We have no idea which one you own, the version of Mac OS installed or any other information.

A pin code to do what?

Is it possible that at one time, you or another (possibly previous and long forgotten) user of that Mac required using a hardware key (USB device) to log in to that Mac?