How Many Tweeters Does A Speaker Need??

One

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.

plenty of speakers with NO tweeters ... depends on the designer and their goals

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.