One of the main questions about ancient societies is how many people lived in them. However, the reliability of population estimates and the methods used to determine them are often overlooked. Many estimates are based on uncertain evidence and can lead to misleading conclusions. For example, popular books like the Atlas of World Population History are built on shaky foundations but are still used as a basis for social science research. Even famous historical figures such as Augustus can be the subject of debate, with different scholars offering different population figures. In general, ancient population figures are highly doubtful and require external confirmation. One method used to estimate population is to look at reported army sizes, but this has its own limitations, such as assuming that the majority of citizens are in the military class. Overall, estimating ancient populations is a challenging task with limited reliable data.
https://acoup.blog/2023/12/22/collections-how-many-people-ancient-demography/