Lucky key is 8


5,1,2,3,7,4,6,8,it took 8 times
3,4,1,6,7,8,it took 6 times
4,5,2,3,7,8,it took 6 times
1,3,4,5,2,6,8,it took 7 times
2,7,8,it took 3 times
5,8,it took 2 times
1,3,7,5,6,4,2,8,it took 8 times
6,4,8,it took 3 times
4,6,3,8,it took 4 times
3,4,1,7,6,5,2,8,it took 8 times
3,1,5,7,2,8,it took 6 times
2,8,it took 2 times
6,8,it took 2 times
4,3,5,1,8,it took 5 times
5,7,1,4,2,8,it took 6 times
5,3,4,8,it took 4 times
7,4,1,2,8,it took 5 times
2,5,8,it took 3 times
6,7,8,it took 3 times
2,6,1,7,5,4,8,it took 7 times

The average is 4.9