Nothing, nothing, nothing can prepare you for motherhood.
I thought I had half an idea of what it all entailed. My sister, who I'm very close to, had 3 under 5 and I spent a lot of time in her house when it was pure madness.
I saw her being quite unhappy, constantly stressed and rushing around between school and play school with a newborn in tow who refused to take a bottle for 8 months.
So yeah, I thought I knew what I was getting into when we decided to have a baby.
I didn't foresee being blindsided by severe PND, but neither did I realise the loss of myself. The boredom, frustration, and worst of all, having to pretend it was all such a JOY because they're only small for a short time and isn't it precious and yadda yadda yadda with all the other ridiculous statements people come out with under the assumption that you LOVE it and it's ALL you ever wanted.
My DC is 4 now and I love every bit of him, he is the funniest, cleverest, huggiest child ever. I love having fun with him.
I also love working full time, I love my own space and my hobbies and he'll be our one and only.
I know myself very well. I know I'd be a shit, shouty, frustrated mother of more than one child.
It's not what we 'planned' pre-DC, just the one, but living it as opposed to imagining it nailed the answer to the question of a bigger family.