"몇 호선으로 갈아타야 돼요?"

Translation: Which line do I need to transfer to?

[rkirsling]

Note that this is asking for a number because Korean subway lines are generally all (#)호선 號線.

[RaphalLefo1]

The translation sounds unnatural

[RQZ.Sash]

It seems that there is a P sound between 몇 and 호...

[oee16]

"Which line should I change to?" would be a more accurate translation.

There is a different expression for "need to", isn't there? Maybe,

몇 호선으로 갈아탈 필요가 있습니까?