If you want an Integer (whole number) random number in a range, you just generate a non-integer random number and then
keep the integer portion of it, using another Math function:
var thing = Math.floor(Math.random() * 20);
After this line of code, the 'thing' variable might contain values similar to the following:
Not 0.19287346, but 0
Not 7.5243, but 7
Not 19.993873, but 19
Not 3.00003873, but 3
In other words, we get whole numbers between 0 and 19, inclusive.
If we wanted between 1 and 20 inclusive, we'd use this:
var thing = (Math.floor(Math.random()*20)) + 1;