By being realistic and cautious about my abilities to cope, provide and include "sparkle" (stuff and emotional availability) beyond the basics.
I worked out not what I could manage if things stayed the same, good. But what the above would look like in the worst of times. Job loss, illness, unexpected divorce, deaths in the family, national economic downturns, my industry hitting road bumps... those sorts of "unpredictable in the longer term" factors.
Because of my experiences in my own formative years in the parent/child equation I wanted to be sure I wasn't basing reproductive choices on just the rosy, hoped for future.
But any future.
Because most people experience some life rain, and some get life hurricanes.
When parents don't cope so well with that sort of life weather - sometimes because the burden of parenthood is too much when multiplied over several mouths to house/feed/attend to/be emotionally available to positively - the kids can take the brunt of the storm.
I wanted to insulate my offspring against my potential feet of clay if the going got tough and I suddenly discovered I was neither tough enough, or get going enough.
Being let down and left to get drenched as the kid was bad enough. But being the parent in that equation, thus making us a multi-generational family of fucking up kids via overly-optimistic forecasts of what is to come and how well one will cope with it, that looked like well beyond what I wanted for my own teeny tinies.
So I stuck with one. He is 18. There have been storms. We've weathered them, in part because one kid was within our tolerances of pressure in crisis. I have no regrets. I am happy I went with under estimating, rather than over estimating my own capacity. Despite loving babies with every fibre of my being. Or maybe because of that. God knows.
We all take our personal priorities and work out how we want to cut the cloth we got given, into a coat we can afford, economically, mentally and emotionally. I guess the trick is to work out what you have to work with in good times and bad, and then look at the "kids needs will always have to be the priority over adults wants, including wanting to curl up into a ball and give up when things turn to shit" baby head count that will work within those confines.
It's always going to be very personal, highly individual maths.