What number is XX?

X when used as a number is typically being used as a Roman Numeral. That numbering system is only used in limited situations, and typically to represent dates such as copyrights or the number of an event. For example the 29th Modern Olympic Games XXIX. Most of the world, when working on the international stage, use English as a standard language. The English language uses "Arabic" Numerals (1,2,3...)

In Roman Numerals you add numbers together unless a smaller number is to the left of a larger number. Then subtract the smaller number from the larger number immediately to its right.

I = 1 V= 5 X = 10

XIX = 19 XX = 20 XXI = 21