No plastic or tarps on the bottom. It will hold moisture and they hay will wick that up .
It depends on your bale size and weight and what type of hay., as an example per horse, I get alfalfa 120 lb bales, heavy bales , not light packed bales, and one bale last about one week.
So I figure 84 bales for 4 months , plus add a few in case of mold in case a cover leaks and in case of rodent damage. Lighter packed bales I try to figure and extra bale or two per month.
Last year we got 42 tons of hay for 18 horses , August to end of July . I just had to get another 100 bales and it should last to the end of Sept, and I am hoping hay prices drop. Usually does as the protein level drops and the dairy people don't want the hay.