The question asks if Brazilian Jiujitsu is appropriate for defending against multiple attackers.
First, just so we're clear, no martial art is good at defending against multiple attackers. That's because it's hard enough to fight one person. When fighting two or more people at the same time, you're probably going to lose. That goes for the best trained people in any martial art, period. So it's a complete myth that any martial art is actually good at fighting multiple attackers. Don't believe anyone who tells you otherwise.
That being said, which martial art do you think would give you the best ability to get back up on your feet if you were in a scuffle, fell down, and found yourself with some guy on your chest ready to rain down punches while another guy was getting ready to kick your head in? The answer is Brazilian Jiujitsu. That's the martial art you want to have if you ever find yourself needing to get someone off of you and get back on your feet the fastest.
BJJ also gives you take-down defense that most non-grappling arts don't have. You need to train in a martial art that tries to take people to the ground, because only in that martial art will you learn the best ways to resist getting taken to the ground.
And if you're taking a non-sport based BJJ such as Gracie Jiujitsu, you'll probably have learned how to punch and kick as well as how to avoid punches and kicks.
In BJJ, you can also use your skill in grappling to maneuver your opponent in order to shield you from other people trying to kick or punch you. If you have no grappling skill, you won't have this ability.
In a similar vein, a BJJ fighter has more of a choice when to get up more quickly and easily. Whereas, people who don't train in ground grappling do not have that choice at all. And it's usually not by choice that people end up on the ground in the first place.
There are plenty of non-grappling based martial arts, such as Karate, Kung-Fu, Wing-Chun, Taekwondo, etc. which all claim to have the "multiple attacker" problem solved in their own ways. The problem is the fine print. It only works in very unrealistic conditions. For Taekwondo to work, for example, you need to be able to move around freely and quickly so that nobody can grab a hold of you, which rules out most situations that don't involve wide open spaces. And if you do allow someone to grab you, it's considered your fault, not the martial art itself.
Most martial arts don't actually train for multiple attackers, either. If you've never had to spar two or more people at the same time, you simply won't be prepared for it when it comes up in real life. You need partners who are really trying to punch you, kick you, grab you, choke you, throw you down to the ground, etc.
What I tend to see in a lot of martial arts such as Karate, Taekwondo, and Aikido, is that they pretend to spar in multiple attacker scenarios. But the attackers tend to attack one at a time. Or they limit it to one strike, and then they'll stop and let you do whatever you want to them. That sort of drill really doesn't prepare you at all for a real life encounter with multiple attackers. It's worse than useless, because it gives students false confidence, which could end up getting them killed in real life.
I have seen some BJJ schools practicing with two people against one, but unless there's a significant size or skill advantage, the defender will almost certainly lose. That's how you can tell that they're doing this kind of training right. Any other outcome would be unrealistic.
All of that understood, if you really have to fight more than one person in real life, there are two things that you can do that work more reliably. First is to run. Second is to carry a weapon, such as a knife or a gun. A weapon would help even the odds.
Hope that helps.
