It would seem to be 11.
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Here are a couple of ways to justify the answer "11":
#1:
2 -> 6 is an increase of 4.
6 -> 3 is a decrease of 3.
3 -> 8 is an increase of 5.
8 -> 6 is a decrease of 2.
6 -> 12 is an increase of 6.
12 -> 11 is a decrease of 1.
#2:
2 -> 3 is an (odd) increase of 1.
6 -> 8 is an (even) increase of 2.
3 -> 6 is an (odd) increase of 3.
8 -> 12 is an (even) increase of 4.
6 -> 11 is an (odd) increase of 5.