On most built-in refrigeration it is on the data plate inside the fresh food compartment, near the top on the left or right wall, or behind the upper drawer. On undercounter and outdoor units it is usually behind the grille at the bottom. Wine cabinets carry it inside the door frame.
Take a photograph rather than writing it down. The full number includes letters after the three digits — those decide which parts fit, and they are the part people misread most often.