I was about to create a thread on this myself. I also pictured having more than 1 (probably 2) but for many, many reasons we are firmly OAD. Our child is almost 2.
My mental health - I massively struggled with intrusive thoughts and after a lot of therapy, they've only just started to die down. So much of my own childhood trauma was resurfaced in a way I never could foresee.
He was so colicky as a newborn, the bone-chilling newborn screams and sleepless nights felt like torture. I also had a traumatic birth.
When he wasn't crying, I found the baby stage quite boring and repetitive - he's got way more personality now of course but wouldn't want to repeat.
I gained so much weight when I was pregnant and it was really hard work to lose it again. My breasts have changed completely and aren't what they were. I'm also still breastfeeding, which I love, but my body has suffered because of it a little.
Returning to work after mat leave was hard - really hard. Manager who didn't give a toss about me, whole team had changed, was up for a promotion and then told I couldn't expect to get one if I'd gone on mat leave for a year.
Generally career-wise - there is some overtime expected in a creative role like mine which would be way harder with multiple children. It's much more common for mothers to also be full time workers and I find that balancing act still very very challenging.
Finances - we're comfortable enough right now and can give him whatever he needs and do fun days out. With two it would be so much harder.
Also - it is bloody HARD WORK. Toddlers are hard work. I am committed to responsive / gentle parenting (not 'gentle' as in permissive parenting, which it's often misinterpreted as). I want to be the best I can be for him and give him all the love he needs, but boundaries and guidance too. That is work enough with just one child.
The schedules to manage pick ups / childcare gaps / getting dog walks in every day feel like a game of Tetris.
I came from a family of 5 children and I felt we didn't get the love and attention each of us needed from our parents. They were spread way too thin. I have a poor relationship with both of them. With just one child, I will hopefully never make him feel neglected or unloved because parents are too busy with the needs of other children.
I also frequently wonder, how people cope with a toddler and a newborn. HOW. I guess you get through it - but I personally wouldn't enjoy it. And that wouldn't be fair to either child!