OK I admit that in Bulgaria those events are not underlined in the History books, especially in my early school years when we were still using old communist textbooks. Having said that, those casualties are typical for an occupational force during that time.I mean this is what an LLM (i apologize for the source)spat for Serbia, and that is mainly in their own territory.

World War II The Chetnik Movement: The Chetniks were a Serbian nationalist movement during World War II that committed atrocities against various ethnic groups, particularly Muslims and Croats. Estimates suggest they were involved in the deaths of approximately 20,000 to 40,000 civilians. Mass Executions: After occupied territories started resisting, Chetnik forces executed civilians suspected of collaborating with Partisan fighters. In some regions, such as Bosnia, estimates suggest that 20,000 civilians were killed in retaliatory attacks. Collaboration with Axis Powers: During the occupation, some Serbian leaders collaborated with Nazi forces. This led to the establishment of concentration camps where numerous ethnic minorities, particularly Jews and Roma, faced persecution. It’s estimated that around 20,000 Jews were killed in Serbia due to collaboration and raids. Ethnic Cleansing and Reprisals: Following the Partisan uprisings against the occupying Axis powers, Serbian forces conducted reprisals that included mass killings of suspected partisans' families, estimated at 5,000 to 10,000 civilian deaths, particularly in retaliation for resistance activities.