In my reverse order, the last Xehanort (Sora) was left out, because he wasn't collected until MX was shown, so the list goes straight from XII. (YX) to I. (MX), like a clock. I should have been more specific on that case. xD
YX can only move forward, so he can't switch space and time. First from the BBS Time, then between BBS and KH1, then KH1, then between KH1 and KH 2, then KH2 and then the present time. He should collect as many past versions of Xehanorts as he could as time passes and there are only four of them I can think of. Including Apprentice Xehanort, if he really has built his own personality (Braig, Vanitas, Terra-Xehanort, Apprentice Xehanort) or AX could have seeded Isa after MX was back in control.
And the same person could have seeded Marluxia's Somebody and Luxord's Somebody in the present time, otherwise I can't think of which person other than AX / Terra could have seeded them. MX himself wasn't there until the very end.
Yes, Terra-Xehanort and Apprentice Xehanort are the same person, except if AX has built his own personality, but we only know that MX was back in control (eye color).
But without some minor doubles, I can't think of which versions of Xehanort could have been collected else, because YX can only move forward hmm. /:
If Nomura meant by that, then it would be odd, cool, but odd and how would YX do that? Ansem SoD may have told him which Xehanort he should collect. If that's really the case, then those Chi characters can only be collected in the present time in the dream world the same way how YX could collect Ansem and Xemnas. If we combine that info with my reverse order and delete doubles, then we would have this:
1 - Master Xehanort
2 - Xemnas
3 - Ansem
4 - Xigbar
5 - Chi Character
6 - Chi Character
7 - Saix
8 - Luxord's Somebody
9 - Marluxia's Somebody
10 - Terra-Xehanort
11 - Vanitas
12 - Young Xehanort
13 - Exclusive Kingdom Hearts III Character
But then what's the purpose of having time travel. It's all speculation though. xD