Muay Thai is a competitive sport martial art (and Thailand's national sport). If you're interested in self defence, I would personally recommend learning a more traditional style (e.g. Muay Boran). But that's just my personal recommendation - it pays to "shop around" and see what works best for you. Most martial arts school offer at least one free trial lesson.
Fighting against more than one opponent is an EXTREMELY DIFFICULT thing to do (fighting just 1 vs 1 against a competent opponent is jolly hard enough) and is a more advanced form of fighting, and as such most schools won't teach it until you're more advanced. If you're looking for a style that teaches at least the core principles of multiple opponent fighting from relatively early on, then I'd probably recommend Traditional Long Fist, particularly the Islamic version which was used by Muslim warriors as far out as Turkey during the Crusades. They employ wide sweeping/arcing strikes which serve to improve your upper strength and also generate more force to penetrate an armoured opponent. Beware of the "Modern Wushu" styles of Long Fist which are designed more for performance entertainment - they have lots of very pretty acrobatic and gymnastic moves which would look great on film, but otherwise fairly useless in a real fight.
But whatever style you choose, the basic rule with fighting multiple opponents is to 'break them down' to fighting one at a time. Never let yourself get caught between two or more people (although if you do, I find Long Fist has techniques to deal with getting caught in between, see 0:28-0:38 of the 2nd video link above). But it's still by no means an ideal situation.
But no matter what, fighting 2 or more opponents is really REALLY hard and extremely dangerous. Always remember that the best form of self defence is to avoid getting into fights. If you're afraid that a group of people are going to attack you (or even concerned that one person wants to attack you), then Contact The Police. -- you don't need to wait for them to make a move. If you have genuine reasons to be concerned about people wanting to hurt you, you can contact the police and the police may even be able to place an AVO against them on your behalf - at least in NSW they can. For FREE WA legal advice contact Legal Aid;
1300 650 579
http://www.legalaid.wa.gov.au/