Backup password

It won't ask for a backup password every time you backup..

At some point though, you chose to encrypt your backups, which would have required a password.

It could have been years ago.

The good news is that there should be no limit to the number of guesses you get. If you set up a password hint at the time, it should appear for you. so take a stab by entering some of your commonly used passwords or variations of it?

Then to reset the password for future backups, use the link in the Related Article at the top of the page (which you may have already been viewing)

ahh, good! I thought you were gonna say you tried em all and none worked..

When you set up the new password, enter a password hint so it'll hopefully be easier in the future..

I believe on newer macOS that it'll offer to store the password in Keychain if you use that

Read and follow the article linked above.