I am an "only" child and loved it.
Only children are just like every other child - a mix of good, bad, hopes and fears. Some have fantastic memories of a happy childhood and some don't. I was a reader and loved make-believe, my parents encouraged me and I had plenty of friends in and out of school. I had great early-life experiences that would have been equally lovely but very different if I wasn't an only child. How lovely your childhood is mainly to do with how you are parented. I expect if you have struggling and troubled parents you may cling to a sibling and find comfort there.
I understand from others (wider family members and parent's friends) that I was a generous child, and warm and happy - but the credit there is entirely due to my parents who were empathetic, kind and generous themselves. You model behaviour according to early experiences and expectations so there's no reason why this should be different for a singleton.
As a child your understanding of life comes from your family and you assume (for many years) that your life is normal whatever your circumstances. If you can provide a stable loving family life for your little one they will be perfectly fine. I have no problem with being an "only" as there are plenty of positives.
I feel very blessed to have had such a happy childhood and think this is key. I was always secure in the love of my parents and know now how precious I was to them. I only once ever have wished briefly for a sibling and that was when they died within a few weeks of each other. I longed at that point (briefly) for someone close to share the grief of parental loss. But even then, a friend of mine pointed out how difficult it was for her when her mother died because she and her siblings squabbled and argued over just about everything so that it was remembered as a very negative and divisive time.
So in conclusion , sunshinemum if you have one child or ten, it will feel normal. Others may believe what they do is right but of course it's right for them. I did go on to have a large family by British standards and people who never knew me as a child say this is a reaction to my childhood. This is rubbish. I love all my children and obviously wouldn't want them not to exist but actually we were really happy with our first born and could easily have stopped there if we hadn't found child-rearing so unexpectedly rewarding! Having the second child was rather scary and intense - and it seemed selfish to inflict a sibling on our first.
Do what feels right for you. Do not be bullied by the "must have two" brigade. I have another friend who had her second child after intense social pressure and who struggled with her relationship with him for years.
All this is still anecdotal. Do what you feel is right for you and your little family. Don't be pressured or persuaded by others. It is your life. Make your own choices and then parent the best way you can.
Good luck.