i did a 15 hits combo with Liu Kang yesterday. i may be wrong but here is what i think it was:
2, 2, 1, CS, 1, F + 1, F + 1, DF + 3

Ok, that's totally useless if you don't know what style to start in.
Here you go, the best Liu Kang Combos I've found so far.
High Hit Count:
Jun Fan: 1,1,F+2,1,1,D,F,3 (13 hits)
Pao Chui: 4,1,1,U+2,U+2, D,F,3 (13 hits)
Jun Fan: 1,1,F+1,1,1,F+2,D,F,3 (14 hits)
High Damage:
Pao Chui: U+2, 4,1,1,B+1 (5 hits, 42%)
Pao Chui: 1,1,U+2,4,1,1,B+1 (7 hits, 43%)
Jun Fan: F+2, 1, 1, ChangeStyle, 4,1,1,B+1 (7 hits, 45%)
Jun Fan: B+1,1,1,CS,4,1,1,B+1 (7 hits, 48%)
Nunchaku: B+2, ChangeStyle, 1,1, ChangeStyle, 4,1,1,B+1 (7 hits, 51%)
If you're trying to get the 12 hit combo for the Johnny Cage Konquest mission, you can use any four hit popup combo with the bicycle kick.

thanks a lot man!

i have trouble doing my bicycle kick from the combos, how do you time it?

Timing is something you really can't explain.

Time things like you time the beat to a song.
You just know when to do it.