I think that having at least one extra mag (loaded) should be standard practice. This is regardless of initial mag capacity. You never know when Murphys Law is going to slap you in the face. Of course, I download all of my single stack mags by 1 round and any double stack mags by 2 or 3 rounds. In terms of how many mags should be kept loaded...I guess it all comes down to practicality/need. For law enforcement, I would have nothing less than 2 or 3 loaded mags as spares. For HD, it can vary. After all, swapping out ammo on a monthly basis, then having to fire off such said rounds after a period of time, can become quite costly.
Either way, I keep at least one loaded mag as a spare...regardless of capacity.