The most common sense of it is uncountable, but you also come across it used as a countable noun e.g. The whole situation was a nonsense. See definition 2 here: https://www.collinsdictionary.com/dictionary/english/nonsense

what a load of nonsense! it's uncountable :) ..... it's nonsense!

I do not know its limits but listening to some people speak*, my guess is that it is boundless (ie uncountable). *myself included.

Simply "It is nonense." You can't count nonsense :)

We would say "It is nonsense." Nonsense acts like an adjective here, describing "it."