in a semi auto pistol the most common failures are magazine issues. It is always wise to be able to swap mags. That being said...I carry the mag in the gun and two spare mags.
My Glock23 .40 has a high cap Mag. Since the Mags loaded are a lot of weight I seldom carry extra,since getting my .380 PPK/S it's Mag. only holds 7rds so I have extra Mags for it, they are small and thin and very lite so I stick one in my pocket.