What number comes in the series 110-20-12-11-10?

The answer is 6.

because the sequence is all the same numbers, but they are in different number bases.

110= 6 in binary

20= 6 in base 3

12= 6 in base 4

11= 6 in base 5

10= 6 in base 6

all the rest of the numbers in the pattern would be 6, because we passed base 6 and all the other number bases have the digit '6' in them.