If you want another child, go ahead and have one. But only if you know you can love them equally.
I am thankful that I had a younger sister, even although the age gap and our mother's habit of favouring me above her made life difficult for us both, both as children and adults.
During my working life as a teacher, I have taught "only children" children with one brother or sister, and children with more than one sibling.
Sadly, a lot of only children have a hard time learning to share, to show consideration to others, and even to do as they are asked. Many of them suffer as they grow older from their parents' entire interest, love and ambitions on their behalf being concentrated on them alone.
Children with siblings tend as a rule not to have these problems.