Question: how many different combinations of 5 numbers 1-20?
Answer: You want to know how many ways different sets of five numbers (you say digits, but since they go up to 20 I think you mean numbers) out of 20. The answer is called “20 choose 5” and it equals 20! / [5!*( 20 = 5)!], or written out 20*19*18*17*16/(5*4*3*2*1). It's 15,504.