Forgot my MacBook PIN number

If you see the first screen that rcosta887 is referring to then your Mac is using a firmware password. You will need to enter the password you established for this. If you forgot this password, then the following article may help:

• How to set a firmware password on your Mac - Apple Support

On the other hand, if you are seeing the second screen, then the Mac is asking you to provide the PIN code that you set using the Lost Mode feature of Find My Mac. You can look up your passcode for a limited time. If you forget your passcode or can't verify your identity to see it, take your Mac to an Apple Authorized Service Provider along with your proof of purchase.

Does this appear when you first turn on the MacBook within about 10 seconds or so, or much later?

Does it look like this 1st screen?

Or is it more like this 2nd screen?

If it is something else, can you take a screen shot?