Confused about 개나 in 사과를 몇 개나 샀니?
I saw this today 사과를 몇 개나 샀니? How many apples did you buy? I'm a little confused about 개나. I think 개 is a measure word, but I dont understand why it would come after how many why not before or even after apple, and exactly what is 나 doing in here, what is it's purpose?

None of the explanations in the breakdown seem to me to fit here ( 1. I, me (casual) 2. (particle after word to add meaning of 'however') 3. (particle after noun to add meaning of 'as well', 'too') 4. (particle after word to add meaning of 'so much') 5. or 6. (particle after verb or adjective to make question, casual).

Thank you.

Question

Confused about 개나 in 사과를 몇 개나 샀니?
I saw this today 사과를 몇 개나 샀니? How many apples did you buy? I'm a little confused about 개나. I think 개 is a measure word, but I dont understand why it would come after how many why not before or even after apple, and exactly what is 나 doing in here, what is it's purpose?

None of the explanations in the breakdown seem to me to fit here ( 1. I, me (casual) 2. (particle after word to add meaning of 'however') 3. (particle after noun to add meaning of 'as well', 'too') 4. (particle after word to add meaning of 'so much') 5. or 6. (particle after verb or adjective to make question, casual).

Thank you.

Answer

-나 : it is used to represent estimation of quantity. 

A : 사과를 몇 개 샀니? - You must answer the exact number. 
B : 10개 샀어. 

A : 사과를 몇 개나 샀니? - You must answer roughly estimated number. 
B : 10개 정도 산 것 같아.

Answer

사과를 몇 개 샀니? sounds a bit demanding. Adding 나 makes it sound more casual and softer.

Answer

when you use 나 you dont expect exact amount.. 
몇개 샀어?You need to answer exact amount. 
몇개나 샀어. You can answer like. . A lot 많이, or  대여섯개 five or six. etc

and 개 is 'bound noun(?)' and this need Number... always come after number.. 

but in this sentence "How many apples did you buy? --> 사과를 몇 개 샀어?" 
There isn't any number but we know '몇' should be replaced to Number when we answer.. that's the reason 개 is come after 몇.... 
good luck!!