It's not always just about hitting your target, but actually stopping the threat if you do have to shoot. You also can't rely on there being only one target, either. Scavenger animals tend to travel in packs, regardless of how many legs they walk upon. 19 rounds is probably more than enough to stop one individual, but increase it to half a dozen and I wouldn't be so confident.
My wife shoots, but does not carry. As such, we really don't train together for hostile shooter situations. I can only rely on someone else to watch my back (and in turn watch theirs) if we train together. Do you and your wife train together?
Not trying to pick on you specifically, just pointing out one of the reasons I carry a spare magazine, and when finances allow, will carry a back up.