I usually bring two, but if/when I bring more, I try to stay consistent with the manual of arms during any given practice/range session.
(Personal opinion to follow) I understand variety is the spice of life, but looking at JustinBiscuit's photo, I see six very different pistols:
1. Sig 226 - traditional DA/SA, no external safety (+ a weaponlight)
2. some sort of revolver with an exposed hammer - DAO
3. sort sort of 1911 - SAO only, grip safety + sweep-down-to-fire frame-mounted thumb safety
4. Beretta 92FS Inox - traditional DA/SA, sweep-up-to-fire slide-mounted safety
5. some sort of HK - traditional DA/SA with a sweep-down-to-fire frame-mounted thumb safety
6. sort sort of Springfield XD - striker-fired "DAO" (sorta), grip safety
If you're shooting for fun -- that's a nice collection! But if you're practicing for defensive proficiency, isn't going back and forth between all those different types of safeties/triggers a bit much? I don't get it. It's like a NASCAR driver going to the tracks (his version of the range) and driving not just the car his life depends on, but lots of different types (SUV with automatic transmission, tiny little hybrid, luxury sedan with manual transmission, etc) all in the same practice session, one after another. Again, as a collection/hobby -- I'm jealous. But what's that saying about fearing a man with only one gun?