이게 한국어로 어떻게 말해요?''how many floors the building have?(in all)
I've learned how to say what floor/which floor something's on'' (책방이) 몇 층에 있어요?
and answer by saying 2층에 or 5층에 있어요 etc.

But how do I ask someone '' How many floors are there?'' all together
and how to respond by saying it's 20 or 40 floors. ''this building has this number of floors.''
건물은 몇 층 있어요?
20층이에요. 맞습니까?

And would this be okay to say ''얼마나 많은 층이 있어요?'' 이 표현이 자연스러워요?
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Question

이게 한국어로 어떻게 말해요?''how many floors the building have?(in all)
I've learned how to say what floor/which floor something's on'' (책방이) 몇 층에 있어요?
and answer by saying 2층에 or 5층에 있어요 etc.

But how do I ask someone '' How many floors are there?'' all together
and how to respond by saying it's 20 or 40 floors. ''this building has this number of floors.''
건물은 몇 층 있어요?
20층이에요. 맞습니까?

And would this be okay to say ''얼마나 많은 층이 있어요?'' 이 표현이 자연스러워요?
좀 도와 주세요

Answer

you can say like this also:
- 그 빌딩은 몇층짜리에요?
- 몇층짜리 건물이에요?
- 몇층건물이에요?

Answer

이 건물은 몇 층까지 있어요? 
- 20층까지 있습니다.

or

이 건물은 총 몇층인가요?
- 20층입니다.
- 20층까지 있습니다.

얼마나 많은 층이 있어요? sounds a little awkward although it is grammatically correct.