If you were taught to say "so many/much" in affirmative sentences INSTEAD OF "many/much," then you had a bad English teacher! "Many" and "much" are fine all by themselves—just make sure to use "many" with countable nouns, and "much" with uncountable nouns. Adding "so" just makes the "much/many" stronger; it is not a substitute (imagine if a teacher told you that you could not say, "I'm hungry," but had to always say, "I'm so hungry"!).