So you currently study a practical self-defense system like Krav Maga. Awesome! You now have all of the tools necessary to defend yourself. Do you need to study kata as well? Nope. Should you do it anyway? Oh heck yeah!
Why? Because practicing kata will:
1) Improve your focus and situational awareness
2) Improve your flexibility, breathing, endurance, and fluidity of movement
3) Will show you creative ways to apply your self-defense techniques
In other words, it will allow you to use your combatives more effectively!
Studying kata makes you the TEFL teacher who has a master's in Literature, or the engineer who has a doctorate in Physics. It gives you that "extra edge" that allows you to implement your practical skills in new and innovative ways. Not to mention that you will be better prepared for the rare scenario in which your assailant has been trained in modern combat or martial arts.
So how many kata should you study? At least four: two from a hard style like Shotokan and two from a soft style like Kung Fu. A hard kata will show you powerful, linear attacks which will teach you how to generate knockout power. A soft kata will teach you fluid, circular attacks which will show you how to use your opponent's strength against them. Learning two of each is important for the sake of variety. At the same time, you don't want to overdo it. Studying too many kata will interfere with your practical self-defense training and ultimately lead to burnout. Unless you plan to teach martial arts for a living, or it is a fun hobby for you, I recommend that you cap it at seven kata. That way, you can do a different one each day, enough to keep it interesting. So as soon as you have a solid foundation in practical self-defense, find a traditional martial arts school and study the kata. Doing so will improve your technique in ways that you can only imagine.