I've never liked that saying. Nine women can make nine babies in nine months, you're just running up against fundamental limits with the nine month thing. Of course there are fundamental limits in software engineering as well, but they're much more difficult to identify than the length of time it takes to make a baby. The argument that fundamental limits exist is not an argument not to try and get to those limits.
Yes nine women can make nine babies in nine months, but they're all going to be born in 9 months (give or take), which doesn't help you have a baby in 1 month. If you plan it right, you can have 1 baby a month with a 9 month delay, but again that's 8 months too late for a deadline.
The saying here is essentially akin to too many cooks spoil the broth. Put it this way: 1 brick layer can lay 9ft of brick in 9 days (no factuality here). If you're building a wall you can only go one row at a time. 2/3 brick layers will speed up the process. However 9 brick layers won't produce a 9ft tall brick wall in 9 days because 6 brick layers will be sat on their arses drinking coffee.
9 brick layers can produce 9 9ft walls in 9 days, but won't be producing a 9ft wall in a day 9 days continuously working as a team.
The statement doesn't preclude that work can be produced efficiently, for example you could easily have 4 brick layers build 4 9ft walls - AKA a house - in 9 days.
The aim is to find your fundamental limit and work with it. If you need a baby every month, sure plan ahead and you could have a baby monthly starting in 9 months, if you can account for that 9 month waiting period. You will never get a baby in 1 month with a team of 9 women and only 1 of them being pregnant.
