The answer to this tricky puzzle is the number 2.
The solution to this puzzle trick is not related to any mathematical equations. This is how you came up with the following answers: The numbers 9999 = 4, 8888 = 8, 1816 = 3 and 1212 = 0 is because you counted the round shapes on each number. The number 9 has one round shape, the number 8 has two round shapes, the number 6 also has one round shape while the numbers 1 and 2 doesn`t have any round shapes.
Therefore, the answer to 1919 is equal to 2 because there are two number 9 with one round shape on each number and the number 1 doesn`t have any round shape.