"How do the numbers 1-20 relate to the array? Or is this just text on the front panel?"
For the front panel control, elements which are labeled 0-20 are placed in that exact order within the array (zero comes first). So if you were to access the 3rd element of the array in the block diagram, it is the exact same number you would see if you selected 3 on the front panel.
If your array size is 30, there will be 30 elements in the array which you can access, which are labeled as array elements 0-29. A very crude way of checking the array size, is to simply use the front panel control. Keep increasing the array index position until you see elements which are greyed out. This means you have gone past the actual size of your array. (Ex. The last element which is enabled, not grey, is number 54. Then your array size is 55, remember to count element 0).