You need at least 40 medium-sized apples per gallon, so if you want 5 gallons, that's like 200 medium apples (if they're small you might need 300 or whatever).
As kramerog alluded, apples and pears taste better slightly softened but are easier to juice when they still have some structure left, i.e., before they turn mealy or buttery. I would advise you not to pick the fruit until many of them start falling off the trees, and then leave them to sit in your basement or in a cool shady place (but not your refrigerator) for about 7 to 10 days before juicing. Pears might need to be juiced a little before then, before they get super soft and buttery, otherwise they'll just be a mess.
You also need a scratter or something to grind up the fruit into little bits. A food processer could work for small amounts but for hundreds of fruits you'll want to get something bigger. Don't just press the whole fruits, you won't get much juice that way! But you probably knew that already.
Good luck and happy cidermaking! I'll be making some myself next month, my apples aren't quite ripe enough yet.