In the original Keyblade War, the warriors crafted Keyblades in order to fight to obtain the unseen realm of Kingdom Hearts. The guardian of Kingdom Hearts, the X-Blade was destroyed in the process, shattering itself into 20 pieces, 7 of Light, and 13 of Darkness.
I think that's it, I can't remember the exact details but the scene explaining this is near the start of the game in the 2nd flashback I think
But the keyblade war happened ages ago and all the keyblades after then must have been created by the X-blade's light and darkness which means that there could only have been 7 wielders simultaneously (with keyblades of light) and that their keyblades would disappear if they died and it would also break the rule that each key of light have their counterpart in the realm of darkness which was established in the Xehanort reports in BBS.
Or, the keyblades of light (and darkness I suppose) from the X-blade could be spread out through history and perhaps none of Sora & co have them.
One really crazy theory I have is that sense there are 7 lights and 7 PoL then wouldn't sort of make sense if these princesses had the keys? Or (I'm making this one up as I'm writing) they are the key to unlock the keys of light from the X-blade, meaning that everyone who has had contact with them and later received a keyblade might have a keyblade of X-light (do anyone els think that's a good nickname?)
*
*
*
*
If we go with the first theory that opposes the fact that all keyblade have counterparts then this is my theory
7 X-lights
Sora/Roxas/Xion, Riku/Terra, Kairi Aqua, Ventus, Lea and finally Eraqus
after Terra was possessed by Xehanort his and Eraqus keys still had their power and all because they where still connected to their hearts. that's how Eraqus key could be used to lock LoD away and Terras key helped save Aqua in RoD (before KHI)
Riku was suppose to get a keyblade but Sora got it instead so I suppose that Riku is using Terras keyblade just like YX used MX's key.
now in 3D there are more then 7 keys being used.
Mickey is using a key and Sora is using 2.
We know however that Sora is using 2 keys because one of them is Ventus but how come mickey can have a key if there can only be 7 keys at the same time?
answer: he don't have a key of light he have a key of darkness, in fact we already know this, he got it in KHI and he have never dual-wielded.
I think the reason he got it is because he lost the power to use his old keyblade but I don't know why it looked like KK and there still wasn't 7 keys in use back then so who knows. Master Yen Sid probably lost that power too.
even if I'm completely lost on all of this I think we can all agree on the fact that Mickey does not have a key of the X-light because he have a key from the RoD