Demographers estimate how many people have ever been born — since Homo sapiens appeared about 200,000 years ago. The method: population benchmarks across eras and birth rates between them.
A number is computed from the date of birth, not issued first come, first served — so everyone who has ever been born has one: ≈ billion records already belong to those born. The ceiling is 200 billion: the table's capacity, with room for more than five centuries, not a forecast. The numbers of those already born will never change.
This is an estimate, not a census: for most of history there are no data, and the further back, the wider the assumptions.
The counter moves because real-time births are added to the estimate at the UN rate — about four per second.
The number is computed from the date and time of birth. Without the time, the place within the day is chosen among that day's free numbers. No number is issued twice.
Source: Population Reference Bureau, 2022