2 6 3 8 6 12?

Looking at the differences between the nubmers in the order given gives this list of numbers

+4 -3 +5 -2 +6

There are alternative negative an positive numbers.

One could say the positive numbers are going up one at a time while the negative numbers are going down one at a time.

If that is the case the sequence needs the next negative number which would be -1 so 12 -1 = 11