When you are training for the ride, you'll have time to see better what works for you. It depends on how long it takes you to ride the distance. I typically ride in the 13-14 mph speed range so wouldn't need a snack for a 20-mile ride so long as I'd eaten beforehand. I will take a snack for a ride for over 90 minutes. You might be fine with a sports drink in your water bottle instead of water. As a new cyclist, if it were to take you two hours to ride the 20 miles, you should stop midway for a snack. A granola bar or a couple of cookies or a "Gu" or similar gel will probably do it for a ride of that length. No need to overthink it. Rides that take all day require a more "finesse" strategy and a lot more food, but you'll be fine keeping it simple on a 20-miler.

Best of luck with the ride and welcome!