Red John is not a person. It is an Organization.  

Why he is many?
It is an ultimate trick in purpose to defend the Organization from CBI and FBI.
It is because of distraction, and removing conclusions about what is the main point.
Red John is flexible Team, based on clear rules and NO HIERARCHY.
If Jane catches one of them, Red John will still exist.
Organization have assistants. They are all, as a rule, seduced. But it is most important that they dont know what is Red John exactly. Even Red Johns victims dont know what is the big picture.
This is the reason why someone like Jane is struggling and why he is helpless in his battle with Red John. Members in Red John team have trust in each other. They cooperate and have divided roles.
It is the killing machinery, consisted of highly intelligent, strong and powerful people.
They have similar ideals and principles.
They are enjoying in their Game. They are all psychopaths, like Jane. But they are evil psychopaths and threat to society. 
Organization is dividing suspect from one to another member in purpose to distract Jane.
It is a reason that we have so many potential suspects,

Remember these Red Johns words?  
"But if you really did have a change of heart, what if you and I were friends? Imagine the life we could lead. It's a higher path, Patrick. A nobler existence."
They want Jane in their Team! They want to defeat their biggest enemy attracting him.
They are using Janes "powers" to impress him, in purpose to force him to submit to them.

For me, it is theory that cover all logical gaps which appears in resolving who Red John is.
Maybe I am wrong. Maybe my theory is so apstract and silly.
Maybe I dont want to see Brett Partridge is Red John. 
One thing is sure, these spectacular series deserves spectacular theory and spectacular finish.
I dont want to be disappointed!