Need tow photos. See:

https://www.uscis.gov/forms/citizenship-and-naturalization-based-forms/document-checklist-form-n-400-application-naturalization

All applicants must send the following 3 items with their N-400 application:

• A photocopy of both sides of your Permanent Resident Card (formerly known as the Alien Registration Card or "Green Card"). If you have lost the card, submit a photocopy of the receipt for your Form I-90, Application to Replace Permanent Resident Card; and
• Two identical color photographs, with your name and Alien Registration Number (A-Number) written lightly in pencil on the back of each photo. For details about the photo requirements, see the N-400 form instructions - or Part 5 of Form M-476, A Guide to Naturalization. If your religion requires you to wear a head covering, your facial features must still be exposed in the photo for purposes of identification; and