Grammar
N뿐이다
only, nothing but
Form
Noun + 뿐 + 이다 — “only, nothing but.”
Function
Restricts a quantity or set to the named items, excluding any others.
Why this form?
-밖에 없다 — “only” requires a negative predicate, unlike N뿐이다.
How to build it
Noun + limiting particle 뿐 + 이다: 명뿐이다 (“only ... people”). The -이- belongs to 이다; *명이뿐이다 is not the intended structure.
Forms not to confuse
-밖에 없다
세 명밖에 없어요. — There are only three people.
Examples
그럼 우리 7(일곱) 명뿐인 거네요. — So there are only seven of us.
이것뿐이에요. — That's all.