Each WvW arena is made up of four huge maps. Each map is capable of holding more than 100 players from each of three factions (servers), which means that upwards of 1,200 concurrent players will be able to get in on the large-scale action. The beauty of having three teams duking it out is that things tend to balance themselves out naturally. If one team goes beast-mode and starts making life miserable for everybody, the other two teams can naturally gang up to fight against the first.
Although lore justification might be a little tricky, in a world with as much magitech the Guild Wars universe already plays host to, it's not too hard to imagine that people from alternate versions of the same world get spat into yet another mutually accessible copy of the world, where their first inclination is (naturally) to start beating the crud out of each other.