If you are going to take on the job of passing on a ryu then all the KATA of that ryu are a requirement.GICHIN FUNAKOSHI when he introduced karate to JAPAN knew only the kata he was taught by ITOSU the 5 PINANS and NAIHANCHI SHODAN .He learnt the other 20 KATA from other masters .He changed movements in many KATA and renamed them .
If your intent is to become proficient in self defense but not to take up the task of passing on the ryu a high degree of proficiency and intense study of as little as 3 KATA will do.
It has been shown the 5 pinans (heians) contain everything you need for a complete self defense system.
CHOKI MOTOBU is said to have known only 4 KATA his favorite being NAIHANCHI (TEKKI SHODAN) and few had his formidable reputation as a fighter.
There is a law of diminishing returns in effect when learning to many kata and trying to have each one have the same value to you as a practioner .Technique are repeated over and over throughout kata that are exactly the same and mean exactly the same thing with the same possible variations.Fighting isn't really all that complex that you need 26 highly repetitive kata to learn how to do it.
Some masters made collections of kata for fear such knowledge may be forgotten that doesn't make their practice or teaching anymore valid than a master who never practiced more than 4 in his lifetime and most masters only knew 4 or 5 .
However they knew their KATA to a depth we are only beggining to understand.
Grossman popularized the sheepdog metaphor. The idea is that there are sheep-- generally nice and productive but not what one would call hard core. And the...