When I go to purchase a product from the Enroll Student page, why am I not able to choose the quantity of the product? Why is there a default quantity already mentioned there?

The default quantity is the number of seats you have in the group to which you are trying to add the new Product to. If you have purchased 5 seats in Course1, which has now become Group Course1, and you are adding Course2 to Group Course1 then you will need to purchase 5 seats in Course2 as well as all your students in Group Course1 need to be enrolled in Course2 as well.