The question is: Can you just stick with 1 ADC, aside of being banned, and be successful? Can ADCs even be counter picked? Are there certain niches that other ADCs support?(Aside of Double AD teams with sivir)
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.
Ultimately: How many ADCs should you learn, as an AD carry?
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.