It isn't very peculiar when you take into consideration that six has been a pre utilized number before: six apprentices of Ansem and those same six founded Organization XIII. But I get your point. Also, Darkness did say that, after the Keyblade War, six remained. Maybe there were seven and only six survived? And the MoM can't be a Darkness: the MoM was too young during the Keyblade War. And if he were a Darkness, Darkness would have achnknowledged it by now
Weren't there 8 apprentices? I think you are forgetting Lea and Isa...
People seem to be misunderstanding what I tried to say. I'm not claiming that MoM is one of Darknesses. Instead, the idea is, seven people fought in the first Keyblade War. Six of them fell into darkness and became these shadows, while the seventh discovered the protective properties of a black coat and became MoM.
The number has to be meaningful, because otherwise why would they waste time pointing it out? They left out things like Ven beating the shit out of the Darkness, and a flashback of Ava giving the keyblade to Brain. One possible meaning is that since four Darknesses remain in the data world, there will be four bossfights coming for the Player and friends in the last update. But is that enough to justify it?
I can think of only one place where the number six is meaningful: the disciples of MoM. This meaning comes mostly from the fact that one of the seven sins is missing. Now, I admit that this is very speculative, but on that DR scene with the Queen of Hearts, Xehanort associates the Darkness with a particular sin, Wrath instead of Greed. This has looked suspicious to me from the start, in the sense that there has to be some deeper meaning. It would make sense that each Darkness is associated with one of the sins, and the scene is a clue for that.