Are there too many Champions in the game?
RIOT likes to survey players about this question, but it isn't a good question to ask players.
For one thing, there are studies that examine the question of the number of people or number of things that people can keep track of or function efficiently with. The threshold in these studies is found to be around the 100 mark. You don't actually need a survey to tell you the answer to this question.
Aside from information overload (which has been studied), why would the number of champions be considered too many at any point? In a word, diversity. As long as each new champion holds a unique place all is well, but as soon as the champions begin to blur together, then it it starts to seem strange that you have multiple champions that do the same thing. But this isn't something you need to ask players all that much. Instead, this is a question for developers to answer. Are the developers able to continue to maintain the unique feel of each champion? Can the developers continue to balance uniquely around each champion?
Well, the answer is that we've started to see more and more that the developers can't handle the diversity. The developers have created six classes, there are maybe 3 sub-classes in each class, and the developers appear to more and more be developing and balancing based on these classes. So that's about 18 unique roles in a field of over one hundred champions.
For example, stat binding on items enforces classes over champion dissimilarity. The Mastery Tree is organized around powerful Keystones that enforce a mere nine play styles. RIOT releases champion updates that target champions classes. We recently had a Mage update and soon we will have a Support update. It's perfectly fine to think about the game in terms of classes as a tool to help organize one's thinking about the game, but there is a difference between using it to help organize one's thoughts and using it to develop play style similitude.
Another thing is the repetition of abilities. How many champions have heals? How many champions have hooking abilities? How many champions have global or semi-global transportation? How many champions have dashes or jumps? Every times a new champion comes out that has abilities that are similar to an existing champion in some way, the uniqueness of those champions diminishes. This is subtle death. A new champion comes out that has a similar ability but the overall kit is still "different". Then another champion comes out that sort of mimics another ability, but again it's just the one ability and the overall kit is different. And then this happens again and again, bit by bit until slowly without ever realizing it champions stop being unique.
Why do I bring this up? I was chatting with a Diamond friend and he was saying that pro players haven't picked up Taliyah because there are plenty of other champions that already "do the same thing". I was a bit surprised and I'm not sold that that is the case, but if it is, then it means the champion pool is approaching heat death. To me Taliyah seems unique, but maybe she isn't that unique. Maybe the sameness of champions is starting to become an issue. Maybe Taliyah isn't unique because her class isn't unique or because her abilities more or less preexist in other champions (in slightly different forms).
To conclude, I'd like to return to the question "are there too many champions in the game" and redirect it at the developers: are there more than a hundred champions in this game or just 18 classes? How many champions are allowed to have an ability that mimics another champion's ability (even if their overall kits are different)? Can you handle balancing the games around this many champions? To me these seem like real questions for developers, not players.