They should release more marksmen! Because it seems to me that the marksmen role has the smallest champion pool out of all the other roles. There are like a trillion top/mid laners and a decent amount of junglers that can be played now and support has a pretty big pool of champions that can be played, but with marksmen there are maybe like 6 good marksmen that can be played usually in every meta.
...16 seems like actually kinda enough
It's not even about the pool, its about the viablility. Not every champion in every role is as viable as someone else. There is a reason you don't see Miss Fortune and Quinn in every game but you see Vayne and Jinx in every game. I feel its worse for supports though. Every game is Thresh, Blitzcrank, and Morgana. What happened to Soraka? Zilean? Where is this forever long deserved Taric rework?
The thing is, ADC (and also support) are extremely specialized roles. Top lane can fit basically anyone, jungle has no specific guidelines to characters used other than them being able to farm in the jungle (which isn't tough), and mid's basically only requirement is to be AP, and even that isn't necessary (AP Rumble top, Corki ADC, AD Varus/Jayce mid is balanced in damage type). ADC requires high base AD, physical spells, and just a ton of AA damage, while not being too tanky (Urgot being an exception).
I cant believe u noone mentioned Teemo :( 
The reason is that every other role can be played by any class. Tanks, fighters, assassins, mages, marksmen, and supports can all play top, mid, jungle, and support. That is why it seems like the ADC role doesn't have enough champions.
Feminists be like: "WHAT ABOUT MARKS-WOMEN?"
The only problem with making more marksmen is figuring out a way to make them unique. All of them use their auto attacks to do sustained damage, with some giving burst. They have to give each one to make them more special. Ashe has CC, Caitlyn has a safe laning phase, Draven has crazy flat damage, Kalista has super mobility, and so on.
I think it might be because it's harder to create an "identity" for characters whose sole purpose most of the time is to deal consistent damage to what's in front of them, there's only so many variations of feasibly healthy marksmen-like abilities you can tack on. Notice how in the Champion Concepts forum there aren't really many marksmen as opposed to other roles, that's because there is more flexibility in, say, a jungler or mid laner.
Because it's hard to balance and make a unique ADC.
Instead of complaining, what exactly haven't they done at this point? Kalista was a trainwreck to some, and they still couldn't make her unique since she is a utility ADC like Ashe and Varus.
