In my humble opinion, basically, those are meaning of " enough, lots of, plenty of, many, much". But we don't say " 아! 바퀴벌레! 아! 많이!, because 많이 is adv. 많이 is adv. 많아 is adj. So, you can say " 아! 바퀴벌레 왜 이렇게 많아! ( you should use adj at the end of the sentence.) If you want to use "많이", you should say " 아! 바퀴벌레 왜 이렇게 많이 있어!". Make sense? Cheers!!!!