Just because the default controllers of asp-mvc have singular names, it doesn’t mean you should implement singular form for all your controllers.
The correct answer is: it depends on the default quantity of the resource that your controller represents.
Singular, example the AccountController is singular because it represents actions (action method) pertaining to a single account only.
Plural If your controller contains at least one action method that handles multiple resources in a single request.
Example plural format
users/update/3
The route above makes you think you are editing all users, which does not makes sense if you read it like a sentence. However, if you read your route like query, it will make much more sense.
If we think about it, a route IS a query: {resource}/{action}/{parameter} looks like a query to me.
users/ shorthand of users/all reads “select all users table”
users/123 reads “select single entity from users table”
users/update/123 reads “update single entity from users table”
Major sites use plural format, see sample below
stackoverflow.com/questions <- list of questions (multiple)
stackoverflow.com/questions/18570158 <- individual question (single)
stackoverflow.com/questions/ask <- new question (single)
stackoverflow.com/users <- display list of users (multple)
stackoverflow.com/users/114403 <- individual user (single)
asp.net/mvc/tutorials <- display list of tutorials (multiple)
asp.net/mvc/tutorials/mvc-5 <- individual tutorial (single)
facebook.com/messages/ <- Display list of messages (multiple)
facebook.com/messages/new <- Create a single message (single)
facebook.com/messages/john <- view individual messages (multiple)
I believe that English grammar should be strictly incorporated in every programming aspect. It reads more natural and leads to good code hygiene.