It is common to talk about a stable population that has a total reproductive rate of 2.1 children per woman.
I want to point out that 2.1 is an arbitrary number that cannot be right over time or geography.
Such a number must be vague because women have a reproductive life that begins probably at age 12 and can last in many instances well into the 50s. The number of actual children that a women will have in this period depends on when they begin and when they end. This number changes significantly. In the US, first child average age has moved from 23 to 30 over a few decades.
The number of generations of such women reproducing determines an additional population factor. Reproduction beginning earlier means more generations born in a fixed century.
Then there's the issue of number of fathers who are being reproduced by the children. Multiple fathers require multiple offspring. This relates to the number of women who actually have a father that needs being replaced versus a sperm bank where one father may be replaced many times over.
I have looked extensively at the actuarial efforts to estimate this number for reproduction and have not succeeded in finding a reliable equation with comparable reliable measurements for the variables in the quirky equation.
We can use 2.1 but it is rough not accurate.