Having one can be a great choice. The plus sides of just the one include (subject to as with having more kids good parenting);
Only children generally are much more independent, happy with their own company, less likely swayed by peer pressure
Housing is much more flexible, no need for bedroom 3.
Much closer relationship with parent
Both parents can enjoy everything the child does without being split between 2kidsactivities
Only paying for one child enables better life experiences which you might not be able to afford with 2 kids
Better opportunities to involve their friends in things eg days out/holidays when older.
They can choose who they spend their time with rather than having a sibling (who in reality they might hate) forced on them 24/7.
There’s more time to spend on their needs, social, physical and educational that you just can’t have with more.
Only children seem to work out social interactions more easily, make friends easily and generally better at sharing as they don’t constantly have to fight for their stuff at home.
Downsides inc
All of parents eggs in one basket so to speak
Potential loneliness (but just requires more effort to arrange play dates etc - a child with many siblings can be lonely too. I had a brother but often felt lonely growing up as had nothing in common).
Sole career for parents in later life is often quoted but this is rarely different with a sibling and often worse. My DHs brother lives by parents but due to drug use his dad is still looking after him and I dread the day his dad can’t do this and we are saddled with his useless brother.
Holidays tend to be designed around access to kids clubs etc, we wouldn’t do a cottage holiday without another family etc.
Given the state of the environment I wish many more people would embrace the positives of only children rather than the negatives.