많거든

there are many, there is a lot

→ 많다
많 many, much, a lot of + 거든 Explanatory sentence ending (you see; because)
adjective Rare #18,864
there are many, there is a lot The ending -거든 often gives a reason or adds an explanatory, emphatic tone, like “because” or “you know.”
KO: 시간이 많거든 천천히 해.
EN: We have plenty of time, so take it slowly.
Forms of 많다

왜냐하면 앞으로도 기회는 많거든.

Because there will be many opportunities in the future.

— 퍼펙트 스트레인저 (2016)

할 말이 많거든

I have a lot to say.

— 베를린 천사의 시 (1987)

걔가 겁이 많거든?

He's a real scaredy-cat, you know?

— Kill Boksoon (2023)

우리 여기 사과 상당히 많거든?

We have quite a lot of apples here, don't we?

— 마더 (2009)