N-(이)나-② 와 N-밖에 차이.

N-(이)나-② follows nouns that express a quantity when that quantity is greater than expected. 
N-밖에 follows nouns that express a quantity when that quantity is less than expected.
Also, Its used with negative sentence....
When these forms are used in same state, indicates N-(이)나-② indicates positive thinking and N-밖에 indicates negative thinking..
Ex.
사과가 두개나 있어요I have 2 apples
사과가 두개 밖에 없어요I have only 2 apples

But N-(이)나-② indicates negative thinking and N-밖에 indicates positive thinking.when used with the negative words.
EX. 첫 문장에서만 실수가 두 개나 있잖아요.
There are two mistakes in the first sentence alone.
첫 문장에 실수가 두 개밖에 없어요.
There are only two mistakes in the first sentence.