"Gallons ain't Gallons". Your US gallon is about 4 litres. I use a 5 litre carboy, which we call a gallon. except that an Imperial Gallon is about 4.5 litres. Confused?
Anyhow as we say in Australia "the answer is a pineapple!". In the first instance it depends on the juice yield that you get from your pressing. I need about 100-120 apples for 5 litres (obviously depending on the size of the apples) if I get 50% yield. So, 80-100 apples per US gallon should do the job for you.
On average, my apples weigh around 100g (say 1/4 lb). So, after rounding off the numbers a bit, 10kg (or 22 lbs) should give you something like a gallon of juice. As a bushel is a volume measure it is a bit hard to get a straight mass to volume figure since a bushel will weigh differently depending on the product (I notice that in his chapter on Apple Mills, Jolicoeur works on a bushel of apples being 40 lbs or 18kg, but this probably depends on the size of the apples). This suggests that a bushel of apples will probably contain a lot of air space, so you don't get a lot of apples in a bushel.
On this basis you could start off working on a bushel of apples yielding around two gallons of juice, but it could be a bit different, depending on the size of the apples and the yield you get. And, yes... just scale up the quantity of apples for different volumes of juice.
Still confused?... so am I. Good luck!