How Many Tweeters Does A Speaker Need??

This is a good question. I would like to hear a speaker designer's view. I thought doubling up usually to increase speed/response, in the mid range anyway. I always wondered how exactly those dual mid range cones could move in sync, and what issues that would lead to in terms of clarity.

Forty-two

It's all about meeting design goals. The typical reason to add more front firing tweeters would be to up the sensitivity. Of course there are pros and cons to that approach. Rear firing ambience tweeters are used to put more treble energy into the room, and try to create more of a 360 degree arc of treble. Most rooms are very absorptive of treble, so more high end energy can bring the in-room averaged response closer to flat.

Line arrays are a different design goal that come with their own pros and cons.

The answer is simple, there's no right answer. There are a lot of ways to get to the finish line.

Same as a bird, exactly one.