The only thing that makes me believe there is only one person who is the real Red John, is that Jane believes it too. He always refers to RJ as a "him", and as a single person. He aknowledges that RJ must have a network, but always being certain that there is one person who is the head of this network. 

But let's imagine Jane being wrong about this. In that case, Red John is not necessarily one person, and maybe the man who tied him to a chair is a different man than the one who posed as Roy Tagliaferro, for example. Because what guarantee do we have that Red John is one single person? We don't. 

What strikes me as odd, though, is that this thought doesn't seem to ever cross Jane's mind. Being smart as he is, he would've thought of this; and if he didn't, or if he discarded this idea, it's probably because he knows, deep down, that Red John is one person.

Jane's been wrong about many things, but I don't think he's wrong about this. I think there is one and only Red John. I just need to know who it is. Though I do have a guess about who it isn't: Partridge. 

My guess is that he shows up every once in a while to throw us off, because honestly, he's got no characteristics for it other than having an odd voice. Plus, that guy's quite dumb; and if he's not, he plays it well. Either way, I think it would be way too obvious and boring if it was him.