Facebook Security Site
I hope it will be helpfull for you.
What is Facebook Backup Code?
Back on the Two-Factor Settings page, under Add a Backup, there is an option for Recovery Codes. Click Setup, and Facebook will pop up a window telling you about recovery codes, and click Get Codes. Facebook will give you 10 recovery codes that you can use in an emergency to get back into your account
If you wanna secure your Facebook account with backup code ffill the Google form given.
Facebook Security Site 