The different numbers are the numbers for each size. If you're knitting the very smallest size, your numbers are the first ones in each set.
So let's say, the sizes for the pattern are sizes 32, [34, 36, 38, 40, 42]
If someone is knitting size 40, her numbers are the 4th inside the brackets, as follows:
P13, P2tog, (P20, P2tog)x1, P13.
There are a lot of numbers in the pattern writing....but what will help you is to highlight the numbers that are YOUR NUMBERS for the size you're knitting.
You didn't mention the size you're knitting...so I used an example that size 40 is represented by the 4th numbers inside the brackets and parentheses.