If they are in a group, you can fire on all of them until one is left standing/running away. If he or she charges you still, you can fire.
Disparity of force allows you to use a stronger level of force due to being outnumbered.
I don't know where you got that, but you better hope they run after you pull your weapon if you think that. Because if you leave 4 unarmed people dead in an alley without evidence of battery on you, you're gonna probably get the needle unless they left behind a note or something saying they were heading out to mug people.