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

Estrilda nonnula

Turdus pelios

Passer griseus

Ploceus nigerrimus

Elminia longicauda

Platysteira cyanea

Nigrita canicapillus

Tauraco macrorhynchus

Cinnyris chloropygius

Psittacus erithacus

Lophoceros fasciatus

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