To make a big batch for 8 people, you will need one large watermelon, 1 cup mint leaves (about 1 1/2 bunches), 4 limes, 3/4 cup simple syrup, and 2 cups gin. Add mint, lime wedges and simple syrup to a large pitcher. Muddle together well, fill with ice, and set aside. Working in batches, make watermelon juice using the entire watermelon (broken into chunks). Strain the juice and add it to a pitcher (you should have 4 cups total) with gin. Stir to combine and serve.