Like I said, until you're comfortable with the gun, your worries will weigh on you whenever you carry it. But, you shouldn't over think things. The what if's with multiple gunman are possible, but improbable. If you try to load up for every possible scenario, you'd never be able to carry everything.
Prepare yourself so that you're ready for most probable situations, and then have a back up plan in place. That might be a BUG or an armed spouse, like madmag, or a larger, higher capacity weapon in your vehicle with a plan to fight your way there and either finish the fight or get out of Dodge.
For instance, when I'm off duty, I usually carry either a 5 or 7 shot revolver on me, along with a couple of spare moonclips. However, my duty bag is always in my vehicle and, in addition to things like summons books, a flashlight, spare cuffs, pens, etc., it also contains a Kel-tec PLR-16 loaded with a 20rd mag and 3 spare 30 rounders in the bag just in case. If I can fight back to my vehicle or if I need to bail from it, I can grab that bag and be able to engage threats from across the room to well beyond 100yds. That's probably overkill for your situation, but the point is the same - you just can't carry everything you might need for every possible situation.