The missing term is 7.

The sequence is a simple alternating addition and subtraction of 1 and 2.

1 + 1 = 2 2 - 2 = 0 0 + 3 = 3 3 - 2 = 1 1 + 5 = 6 6 - 2 = 4 4 + 7 = 11 11 - 2 = 9 9 + 10 = 20

Therefore, the missing term is 7.