얼마나 and 얼마

'얼마' is Noun .. means  the numbers or degree that we don't know,
- How much does this bag cost?
이 가방 값이 얼마죠?

the numbers or degree that is not decided,

- Eat them as many as you can, I don't care how many you eat!
얼마를 먹든 상관 없으니, 가능한한 많이 먹어라.

small noumbers or degree that is not needed to be clear.

- I gave some money to that homeless
저 떠돌이에게 돈 얼마를 주었다.
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얼마나 is adverb

expression of wonder or awe about .(how beautiful it is ! 얼마나 아름다운가!).
and
as you know it means how many or how much
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when 얼마나 is used for 'how much or how many ' you need to focus on the fact '얼마 is Noun, and 얼마나 is adverb'

there isn't any adjective just after 얼마.
but when you use 얼마나 .. you need to add adjective or verb after 얼마나

ex) what is the price of this?(이건 가격이 얼마입니까?) and how much does it cost? (이건 가격이 얼마나 합니까?)
both are same meaning.. but grammar structures are different..
it depends on the structure that you make ..
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얼마 is only for quantity, but 얼마나 is for quantity or quality

ex)
how much beautiful, how much fast, how much blue..
얼마나 아름다운, 얼마나 빠른, 얼마나 푸른(you can't use 얼마 in this case)

How much does this bag cost?
이가방의 가격은 얼마 입니까?
이가방은 얼마 입니까?

How many hamburgers do you want?
햄버거 몇개를 원합니까?
얼마나 많은 햄버거를 원합니까?

How many days?
얼마나 많은 날들?
몇일?

How many people will come?
몇명 와?
사람이 얼마나 오지?

How much money do you have?
돈 얼마 있어?
가지고 있는 돈이 얼마나 되?

Source: https://www.italki.com/question/92815?hl=ko