8, 9, 8, 7, 10, 9, 6, 11, 10,?, 12

The given sequence is a combination of three series :

I. 1st, 4th, 7th, 10th terms i.e. 8, 7, 6, ?

II. 2nd, 5th, 8th, 11th terms i.e. 9, 10, 11, 12

III. 3rd, 6th, 9th terms i.e. 8, 9, 10 The pattern in I is - 1.

So, missing term = 6 - 1 = 5.