These are species with at least one recorded territory in Thailand, 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 Thailand, among those a photograph exists for. Most recorded is not most numerous: it rewards animals that are common, conspicuous and live where people go.

Acridotheres grandis

Geopelia striata

Pycnonotus conradi

Rhipidura javanica

Streptopelia tranquebarica

Cinnyris jugularis

Aerodramus fuciphagus

Copsychus saularis

Eudynamys scolopaceus

Pycnonotus aurigaster

Spilopelia chinensis

Coracias affinis
The reference database has no regions filed under Thailand.
The field guide covers Thailand on the map, but none of its species have an entry written for them yet.