An adjective can be changed into an adverb by adding the adjective stem to "-게", but some of adjectives can take "-이" like "많다", "같다", "높다", "빠르다", etc. e.g. 많다 => 많(adjective stem) + 게 => 많게 많다 => 많 + 이 => 많이 ("많이" is more commonly used than "많게") "많다" has two adverbial forms, likewise, 높다=>높게/높이, 같다=>같게/같이, 빠르다=>빠르게/빨리 \^o^/

많니 is for question. 너 친구 많니?=do you have many friends? i 나는 친구가 많이 있어. =i have many friends. 나는 친구가 있어. =i have a friend.

The word "많-이" is an adverb, and the word "많-게" is the conjugated form with the connective ending "-게" of adjective "많다". So, "많게" is not an adverb. However, it can be used in the same meaning of adverb "많이".