Yes, it is reasonable for Brittany to think that she will need more than 2 batches of cookies to feed all of the people at her school.
One batch of dough makes 2 dozen cookies, or 24 cookies.
This means that two batches of dough makes 4 dozen cookies, or 48 cookies.
If Brittany only brought 48 cookies to school, some kids wouldn't get cookies. So, we know that Brittany should probably bake more than 2 batches of cookies.