I understand manga (Japanese comic) can be both single and plural. I also understand that ‘many’ is used with countable nouns and ‘much’ is used with uncountable nouns.
I think manga is countable. So I should say “I read too many manga.”
You are correct in saying, “I read too much Manga.” If you wanted to use ‘many’, I would say, “I read too many Manga comic books.”
Interesting to note though, is the idea of ‘too much’. What is ‘too much’? I mean, how much is too much and who decides how much is too much. I do not know the context of this sentence, but in isolation, I would add more information to your sentence.
I read too much Manga and not enough Science Fiction.
According to my mom, I read too much Manga.