How Many Tweeters Does A Speaker Need??

The $200k Magico Q7 has one tweeter. The $40k Zu Dominance has two tweeters. The $200k Genesis 1.2 has about 20 tweeters. The big Wilson flagships have two tweeters. I'm sure at 750 pounds and $200k, the Magico can fill a very large room with sound. If that's so, why does another speaker need two, three or twenty tweeters in one speaker?
plenty of speakers with NO tweeters ... depends on the designer and their goals

The only reason I see for having more tweeters is to have more output and effectively lower the stress on each driver. But there are a number of issues that are created by running two drivers with the same frequency range.
in general, most designers only add tweeters to balance sensitivity differences between midrange and high frequencies.
A line-array like the Genesis is an altogether different beast than the 2-tweeter models. Line arrays do some cool things -- like losing only 3dB per doubling of distance (as opposed to 6dB), since there isn't significant vertical dispersion beyond the height of the array.

On some of the 2-tweeter models, 1 is a "normal" tweeter and the other is a supertweeter with a high pass of ~14kHz-20kHz that provides extension up to 35kHz - 60kHz (or beyond).

On speakers with 2 tweeters operating in the same range, there's no avoiding comb filtering effects.
And then there are those with a front firing and a rear firing tweeter as does my Dantax.
who needs tweeters. get a panel speaker and you'll get more realistic treble response.
For Maggies it is 'How many linear inches of tweeter" do you 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.
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.