I reckon wrestling is only good in a 1v1 but I might have the wrong conception of it. Anything which teaches you the confidence to quickly take someone down seems the best since you'll probably go up against several people. Bugnir overrating size by exaggerating the amount of +2 meter meatheads walking around trying to rape you. Some 1,60m girl in northern sweden fought off 3 babbar last week. She learnt how to box by her father and grandfather.
Boxing is great, and it works in 1v2x situations... if they are smaller than you/incapable of grabbing you. I've sparred boxers/kickboxers before. Shoot a lazy single and they're on their ass for 5 minutes.
In general, measuring a martial art by "how many people can I take on at once" is a horrible metric, because if you're outnumbered your best bet is to run/scream get help. Especially if you're a small girl. I mean fuck, if someone has a knife or a gun and you try to fight them at all you're gonna get fucked up.
If he's looking for the best martial art for self defense, the only practical situation to applying that is in 1v1 scenarios. Where again I'll go back to my point that Wrestling/BJJ is the best foundation, especially when complimented with Muay Thai (or Dutch Kickboxing)