I carry two mags both are 12 rounders...there are more reasons to reload than an empty mag.
Mag failures say that I need two mags...
You should always carry the same rounds no matter what for self defense, and ensure those are reliable in your weapon. Typically the spare mag is carried for; 1. reloading in the event the situation dictates it, 2. Non-diagnostic malfunction clearing is largely built on the principle that stripping the mag, rack the slide, inserting the new one and racking the slide will get the gun up and running faster and more efficiently than spending time and assessing the malfunction and then decided what to do about it.
Carry a spare.