It is easy to be fooled into believing that you need a dozen black belts to be an amazing martial artist. After all, if you can become a great fighter through gaining one black belt, getting a dozen should make you even better, right?
Wrong! Getting multiple black belts is completely unnecessary for one simple reason: all EFFECTIVE martial arts systems which cover the same aspect of fighting will have similar techniques.
Does this mean that all striking systems are exactly the same? No. Does it mean that all grappling systems are identical? No. It just means that no matter what you train in, a front kick is a front kick and a rear naked choke is a rear naked choke. The name, setup, position, or target may vary. However, the move itself will be more or less the same. This is because:
1) People of all cultures and nationalities have two arms, two legs, two eyes, a neck, and a groin. For this reason, the body mechanics of each move will be similar.
2) The best martial artists cross-train to borrow/steal ideas from each other. For example, Sambo, while incorporating striking and the folk wrestling styles of Eastern Europe, has judo as its base. Judo, in turn, was heavily influenced by Catch-As-Catch-Can wrestling from Europe. Krav Maga has moves from multiple styles, such as karate, Muay Thai, wrestling, and Brazilian jiu-jitsu. I could go on and on, but you get the basic idea: everybody basically copies everyone else.
So how many black belts (or instructor-level ranks) do you need? At most, two: one in a striking art and one in a grappling art. If you are doing a martial art which covers all areas of fighting, such as Krav Maga or Combat Sambo, you only need one. Getting more than two is pointless unless:
1) You are doing this for a career and want to stand out from the competition.
2) Studying self-defense is an enjoyable hobby for you and you are doing this to have fun while meeting cool people.
Under those circumstances, it is worth your time. Other than that, you are better off excelling at two styles at the most.