많이 있어요 is like to say "There are/ I have many somethings" 많아요 is like to say "Somethings are many in someone or something" (this is a way of saying in Korean) For physical/tangible objects, 많이 있어요 and 많아요 are interchangeable. -친구가 많이 있어요=친구가 많아요 (I have may friends) But for intangible objects when you talk about someone' characteristics, we normally use 많아요 only, not 많이 있어요. For example, -저는 잠이 많아요 ( I sleep a lot (Grammatical translation: I have lots of sleep) - 저는 잠이 많이 있어요 is wrong. -그는 말이 많아요 (He's talkative : He has lots of talking) - 그는 말이 많이 있어요 is wrong. Maybe it will be understood that he has many horses.