I am an only child and have just one, 14yo daughter.
She is amazing, privileged in a way she wouldn't be if she had siblings, purely because the money would have had to stretch further, but not spoilt.
We have a very close relationship, and I can honestly say she is one of my very favourite people. As far as I can tell being an only child has had no impact on her.
The only things that have been a bit of ball ache are trips to the park and holidays when she was small where you kind of have to be their companion a bit more than if there were siblings, but I think that also depends on the child. She was just not one for wandering off and befriending other kids, but her best mate was the same and she has a younger brother.
From my POV I have never minded being an only child. I like my own space, possibly a bit too much tbh and never had any problems occupying myself.
As an adult it has hit me more. My dad broke his back a couple of months ago and having to manage his medical needs, support mum, run a home and parent my DD single handed whilst working full time has been fucking horrendous, and I'm currently off work due to stress.
At one point a chap was admitted in the bed opposite my dad. He had 2 daughters and 2 sons all tag teaming looking after him and supporting his wife. One at least one occasion I left the ward in tears seeing the difference having siblings can make. But equally I know plenty of people whose siblings have been worse then useless when the chips are down.
It really is a case of what feels right for you as you just can't predict how having siblings (or not) will pan out.