A stopover or a full week
Many people see Hong Kong on a stopover of two or three days between Europe and the rest of Asia or Australia: the Peak, Tsim Sha Tsui, a night market and a crossing on the Star Ferry. That fits easily in the 3 GB pack, valid for 15 days. A full week, with a day on Lantau for the Big Buddha and a hike on one of the trails, sits well in 5 GB, which lasts 30 days.
Take 10 GB if you work from cafés through the hotspot or share data with someone whose phone has no eSIM. The 20 GB pack is for a month in the city. Top-ups go onto the same eSIM, with no new QR code. The data calculator gives you a figure for your own plans.