Help with generating random number

finke7

So I want to generate a random number between 15-20 (number 1) which will be a number needed so i can launch that many balls into my game and then launch a x2 ball when (number 1) is reached .Then I want to want it generate a new number every x2 ball. I know how to make it work with a single number say, 5 balls until a x2 ball, just not a random number that keeps changing.