"Did he fire six shots or only five'? Well to tell you the truth, in all this excitement, i kind of lost track myself. But being that this is a .44 Magnum, the most powerful handgun in the world, and would blow your head clean off, you've got to ask yourself one question: 'Do I feel lucky?' Well do ya, punk?"
I have +1 in the G19, with 14 in the reload.
It's easier to load the mag on a closed slide.
I've never had an issue loading a full mag in any of my 19's with a closed slide.
That being said, when is the last time your Glock 19 failed to lock back while shooting at the range? 99.9999% chance in an SD shooting IF you had to reload (Most shootings are over in..2-4 rounds I think it was?) your slide would be locked open during the reload.
My LCP is a bit rough to load a mag with the slide closed because I use MagGuts which give me +1 with no baseplate change or extra bulk, but I've trained myself to slam the little thing in naturally and I don't really pay attention anymore. Either way, if I'm still in danger after firing the 8 rounds of .380 in the first place I probably have other issues to worry ahout.
This is how I was taught and continue to train. I carry a G19 with the standard 15 round mag. My secondary mag is a 17 round. I carry the 15 round because it conceals a little easier than the 17 rounder. (I know not much difference, but it’s why I carry a G19). Reload as stated above so that I’m always ready.
To the original question, I suggest carry with a 8 round mag and have 1 or 2 backup mags. Reload as necessary - based on the strategy quoted above. That gives you 9 (8+1) + 7 + 7 = 23 rounds or 2 reloads in case of malfunctions. That should be plenty for any civilian situation.