Assuming that you can insert integers of any size, the complexity will be the same as the complexity of division.
So it is O(log n) due to log n is the number of digits.
Notice 1: If you can only insert 32 or 64 bit integers, the “complexity” will be O(1).
Notice 2: Since computers save all numbers in binary, you can get n % 2^k in konstant time, even if n can be of any size. You just take the k smalest bits. This dont work for n % 20 without computing the representation of n to the base 20.
If you want to know what Big-O means, this post will help you.
solved Big O Of Mod 20