You will need a 19" spare which Ford doesn't make. Fords normal 18" 2015 mustang
spare will not fit...on the front, because of the brakes.
You COULD use the factory Ford Mustang spare (18")but only on the rear. So if you get a front flat, you'll need to jack it up, take off a rear normal wheel, put it on the front, then put the spare on the rear.
They're not expensive ordered at the dealer and fit under the rear trunk mat.
But you'll need two jacks then.