얼마 is used for price. 얼마예요? - how much is it? 얼마나 is used with adjectives or adverbs. 얼마나 커요? - how big is it?

To understand 얼마 is easier. Nomally, 얼마 is used to ask the price of something, 얼마예요? How much does it cost? example, Noun (이/가) 얼마예요? 아이폰 X가 얼마에요? How much is iPhone X? 얼마나 is placed before advers and vers to mean "how", "how much", "how many". 1. example, 얼마나 매워요? How spicy? 이 김치는 얼마나 매워요? How spicy is this Kimchi? 2. example, 얼마나 + adverb 얼마나 자주? How often 얼마나 열심히? How hard 얼마나 천천히? How slow 한국어 공부를 얼마나 자주해요? How often do you study Korean? 3. example, 얼마나 + verb 얼마나 먹었어요? 오늘 빵을 얼마나 먹었어요? How many bread did you eat today? 얼마나 마셨어요? 오늘 물을 얼마나 마셨어요? How much water did you drink today? Hope that will help for your question! good luck, friend!