This is why census and biometrics exists.
“In August 2023, over one million Paraguayans disappeared. Results from the 2022 census, released that month, put the number of Paraguayans at 6.1 million — 1.4 million fewer than the numbers projected by the 2012 census. The numbers left Paraguayan policymakers reeling. As the minister of finance, Carlos Fernández Valdovinos, told reporters, “We will basically have to plan for a new Paraguay.”
Paraguay’s experience is not unique. In 2025, Indian enumerators are planning to conduct the country’s first population census since 2011. Retrospective surveys done by India’s National Population Register suggest that the earlier census could have missed up to 28 million people — roughly the size of the state of Punjab at the time.
In Nigeria, the most populous country on the African continent, there hasn’t been an official census since 2006. That, too, was marred by controversy and accusations of miscounting. As Eze Festus Odimegwu, the head of Nigeria’s National Population Commission at the time, put it, “No census has been credible in Nigeria since 1816. Even the one conducted in 2006 is not credible.””
https://asteriskmag.com/issues/11/why-governments-cant-count