[jdnenej]

It's a copypasta. OP didn't make up the number 2 but I'm guessing it comes down to 2 being a tolerable number to most people.

[yoube]

I mean, it's literally the replacement population. 2 parents die, 2 children remain.

[adrianN]

Technically you need a little more than 2 because people die without having children occasionally.