It is impossible to know what the missing number is; ANY number there would make a valid sequence.
In any problem like this where the sequence of numbers is shown without any information about what kind of sequence it is, all you can do is look for a pattern that predicts what the missing number MIGHT be.
In this problem, there is a nice pattern that can be used to find a PROBABLE answer to the problem.
-8 ___ 6 10 12 <-- the given numbers
___ ___ +4 +2 <-- the differences between successive numbers
Use the pattern of the differences to work backward in the sequence....