Seoul, Busan and Jeju: sizing the pack
A week in Seoul, moving between neighbourhoods by subway with a map app open most of the day, fits the 3 GB pack, which lasts 15 days. Add Busan, which the KTX reaches from Seoul in under three hours, and a few days on Jeju, and you are looking at ten to fourteen days with more photos and more map use: that is a 5 GB trip, and the pack runs for 30 days.
Choose 10 GB if you stream on the train, make video calls home every evening or share data with someone travelling with you. Jeju is a short domestic flight from Gimpo, on one of the busiest air routes in the world, and the same eSIM keeps working there. The 20 GB pack is for a month in Korea or remote work over the hotspot. The data calculator estimates it for your own plan.