Answer: Infinitely many
Step-by-step explanation:
The set of natural numbers is {1,2,3,4,5,...}
Set A considers natural numbers that are larger than 13. So we'll be looking at numbers like {14,15,16,17,...}. There are infinitely many numbers here. There is no largest natural number. So that's why set A is infinitely large.
If we rephrased the "x > 13" into "x < 13", then set A in roster notation would be {1,2,3,...,10,11,12}. This would show that we have 12 items. In this case, we have a finite set.