With 1 child, you can give them 100% of your resources (attention, money...)
With 2, you can’t.
So - which is more selfish?!
I’m 1 of 6, several siblings are no contact.
My child is 1 of 1, and what with life always throwing curve balls, now has 2 sets of step siblings that she’s really close to.
So the difference between 1 and more (siblings) has actually worked out better for her than me!
The only area where I think you have to be careful, as a parent of 1, re selfishness is your child’s responsibility for you as you age. It would be selfish of me to expect my 1 child to carry the full time, financial and emotional burden of that. So I will try to make sure it is otherwise.
My 2 friends who are closest to their parents now (we’re all 50s) with a genuine desire to spend time with them (not duty) are both only children.