That is kind of rhyme 많아 means many 않아 (negative) 잖아 is Suffix or emphasis (negative)

잖아 : You can use this expression as a question tag. isn't it? doesn't it? or haven't I?, firstly when you want make sure something, you can use this expression like this : 이거 맞잖아? (meaning This is right, isn't this?). When you want to say, 이거 나쁘잖아.(meaning This is bad, or wrong, don't you know that?) in this case you assume the listener knows what your talk about and then just emphasize what you say. But be careful you shouldn't use this expression to a person or people especially older than you, or your teacher or your boss, then you can be in a big trouble. 많아. This actually means 'lots of' or 'plenty of', or more than something/someone. etc. 그 사람이 너보다 나이가 많아. meaning He is older than you. 그사람이 너보다 돈이 많아 meaning He has more money than you (or He is richer than you) 그 사람은 친구가 많아. He has lots of friends. 않아. means 'not' 나는 기분이 나쁘지 않아. meaning I am not upset or angry. 그 사람은 일을 하지 않아. meaning He doesn't work. He doesn't have any job. or He doesn't go to work. etc. 난 당신을 사랑하지 않아. I do not love you. If you talk a bit formally and show just a little respect to the listener, you just add ~요? or make a negative question instead of an affirmative sentence. ~않아요? ~잖아요? or 많지 않아요? Even more, a more respect you want to show then, add 습니다. at the end of the sentence. 많습니다. 않습니다. but not '잖아' We don't speak like that. Learning Korean is quite difficult sometimes, even for Korean. Lots of tricky grammars. But there is definitely something that helps you understand Korean better. Keep trying and you can get what you want. (한국어를 배우는 것은 어렵습니다. 심지어 한국인에게도 어렵습니다. 힘든 문법이 많습니다. 하지만, 한국어를 잘 이해하도록 도와주는 무엇인가 반드시 있답니다. 노력하면 원하는것을 얻을 것입니다)