While googling, I have encountered 'too much stimuli.' 'stimuli' is the plural of 'stimulus.' And 'too much' is for uncountable whereas 'too many' for countable, right? I just don't get the grammar behind it. Is it only an idiom like that?
"Too much stimuli" is an error, but it's a common error, and it has a purpose.
The difference between countable and uncountable nouns is part of good English, and learners should always attempt to use good English, but should be aware that one will often hear native speakers making this kind of error: e.g., "Five items or less".
In this case the meaning "stimuli that are too intense" is hard to express. It is different from "too many stimuli", but could be expressed by "too much stimuli", even though that's an error from the point of view of prescriptive grammar.
I would recommend that a learner avoids expressions like "too much stimuli" but should understand them in context. The Googled examples likely meant "too much stimulation" or "overly stimulated." In this (as in many cases) it helps to know the etymology of the word (i.e., Modern Latin, 17th century). Even a cursory knowledge of Greek and Latin root words helps to avoid these mistakes.