I can find this stated for vehicles on page 149 beta rules, but can't find it for personal scale combat.

Do the same rules apply to NPCs? How about minion groups?

Tea

I remember this being covered, though I don't have the page number handy, but yes. I remember it saying you can only have one critical per hit. If you trigger multiple hits from one attack (from automatic weapons, limked weapons, dual weilding, etc), then you can trigger one crit per hit. Otherwise, triggering a critical hit multiple times just adds +10 to the roll per time it's triggered.

At least going by the Beta, you only get a single roll on the Critical Injury (character scale) or Critical Injury (vehicle scale) charts. However, you can activate a Critical as many times as you have Advantage to do so, with each subsequent activation providing a +10 bonus to the percentile roll on the chart.

It's this fact (multiple activations) that makes weapons such as lightsabers (Crit Rating 1), disruptors (Crit Rating 2), and vibro-weapons (also Crit Rating 2) so very nasty in this game, particularly as many of them also have the Vicious quality (which also adds a bonus to the critical result roll). So even if you only rolled a single success when attacking with a vibro-ax, if you've got 6 Advantage, you can spend those Advantage to score a critical hit with a +20 modifier (on top of it's Vicious rating), making it more likely that one hit is going to leave the target in a world of hurt.