What number is the turn signal fuse for a 2004 mustang?

The turn signal fuse for a 2004 Mustang isn't a single fuse. The turn signals are controlled by multiple fuses and relays, depending on which side is malfunctioning (left or right) and if it's also affecting the hazard lights.

To find the correct fuse(s), you must consult your owner's manual. The manual will have a fuse box diagram showing the location and function of each fuse. It will likely specify separate fuses for left and right turn signals, and possibly another for the hazard lights.