If you're going to learn a champ like EZ that's commonly banned, it might be a good idea to learn a backup for the adc position.

ADC's are my favorite type of champion. I really don't think that you need to be amazing at all AD's. I suggest you learn 2 or 3 and become good with them. I also would suggest to stay away from Ashe and Tristana. From my experiences with them they are not nearly as good as the others. Being an AD means that you have to get most the farm and positioning is very important to keep in mind. Everybody should buy wards so it isn't just the supports job to do that. With a lot of AD's such as Kog and Ezreal have very good poking with their abilities and you should attempt to get them to semi low health with your ability's then go in and kill them briskly. It would also be a good idea to steal their blue for you or your support or give it to the jungler. Most of the ADC's are pretty similar in rolls so it really will not be hard to learn a few of them.

You need Urgot, plain and simple

Do you truly believe that the pros master every champ in their role? Most of them only know 3-5 to deal with counter picking.

Do you truly believe that any given pro can't play a large number of heroes in their role to a very competent degree, to the point most people would call it mastery?

Sure, you're not going to see Wickd playing the best Lee Sin in the world, but I guarantee you he's pretty good at Lee Sin, if not 100% completely perfect with him.

Just one.

Welcome to the League of Draven.

You can be successful by doing anything, but you aren't going to have a fighting chance to be successful if you just main one and learn a few more, especially with ADCs (or supports, for that matter). ADCs can definitely be counterpicked with strong consequences and benefits depending on matchups - if anything, they're more prone to the effects of the opponent's picks - because you need to worry about several times as many champ-champ interactions affecting your lane. You need to be concerned with:
How well your ADC synergizes with your support
How well your ADC matches up with the enemy ADC
How well your ADC matches up with the enemy support
and, also, how well the enemy ADC and support match up to your support, and how well the enemy ADC synergizes with the enemy support although that isn't dependent upon your ADC pick.

If you consider ADC to be the main role you play, you should ideally try all of them for maybe a dozen normals, just to get the feel for what they can do when you're playing as them (and what they can do to you when you're against them). I'd recommend mastering 5 ADCs minimum, asap, and expanding if you can.