About a dozen Key limes should yield 1/2 cup of juice. Key limes are only about the size of a golf ball, so they produce much less juice compared to your typical lime. Please don't use regular limes, you will insult the entire state of Florida! Alternatively, you can use bottled key lime juice if you can find it at your grocery store, though fresh is always best.
You can grind your graham crackers in a food processor, smash them in a zipper plastic bag, or buy them in crumb form.