Pros for me of only having one child:
more money, a better relationship with DH (who is DS's father), more time and love and attention to give to DS, more patience to give to DS, generally a much happier and more mentally stable mother (note I am speaking about myself here - not applying this to anyone else!!), I would have to have an elective C section which scares me even though I know they technically are very safe, the baby might be disabled (I'm really really sorry if that offends anyone, but I grew up with a disabled sibling and it was so so hard - my mum is an absolute star and I'm just not sure I could do it myself)
cons: I loved the newborn stage and I am worried DS will be lonely when DH and I die.
Neither of those cons, IMO, are good enough reasons to have another child. Babies grow up, and DS and a sibling might not get along.
You have to do what's right for you.
I do not agree with the saying "you'll never regret having another child, but you might regret not having one." I suspect lots of people do actually feel regret regarding children full stop, but it's so taboo that no one talks about it.