These are species with at least one recorded territory in Hong Kong SAR China, not a national checklist: a bird list from eBird or Avibase is more complete. The counts follow recording effort as much as wildlife, so a country with more visitors and more naturalists will show more of both.
The species with the most open occurrence records in Hong Kong SAR China, among those a photograph exists for. Most recorded is not most numerous: it rewards animals that are common, conspicuous and live where people go.

Pycnonotus jocosus

Spilopelia chinensis

Pycnonotus sinensis

Copsychus saularis

Zosterops japonicus

Milvus migrans

Orthotomus sutorius

Passer montanus

Gracupica nigricollis

Acridotheres cristatellus

Parus cinereus

Garrulax perspicillatus
The reference database has no regions filed under Hong Kong SAR China.
The field guide covers Hong Kong SAR China on the map, but none of its species have an entry written for them yet.