It's important to stress that the game/other sources never insist that ALL member names MUST be an anagram of their original name + 'X'.
I have seven known with no argument, a good guess at an eighth, and no other idea at all.
1: Xemnas - Ansem + X
2: Xigbar - Braig + X
3: Xaldin - Dilan + X
4: Vexen - Even + X
5: Lexaeus - Aeleus + X
6: Zexion - Ienzo + X
7: Saix - ?
8: Axel - In that classic scene where he fights Roxas for the last time, in Kingdom Hearts 2's beginning, and he spins his two chakram on the ends of his fingers, he looks like an Axel and wheels.
9: Demyx - ?
10: Luxord - ?
11: Marluxia - ?
12: Larxene - ?
13: Roxas - Sora + X
I have seven known with no argument, a good guess at an eighth, and no other idea at all.
1: Xemnas - Ansem + X
2: Xigbar - Braig + X
3: Xaldin - Dilan + X
4: Vexen - Even + X
5: Lexaeus - Aeleus + X
6: Zexion - Ienzo + X
7: Saix - ?
8: Axel - In that classic scene where he fights Roxas for the last time, in Kingdom Hearts 2's beginning, and he spins his two chakram on the ends of his fingers, he looks like an Axel and wheels.
9: Demyx - ?
10: Luxord - ?
11: Marluxia - ?
12: Larxene - ?
13: Roxas - Sora + X