We are only having one, but I do sometimes thinks our daughter will be all alone. But then I speak to my husband who is an only child and he says he didn't mind being a lonely child and made up for siblings with lots of close friends. I wouldn't say they will have a harder time being an only child. X
I have 2 with an 18 month age gap, some things are harder, especially when they run in opposite directions at the park but they also make being at home easier as they happily play together and have a ready made best friend in each other.
This is me 100% genuinely don't think my mental health could take having another. Its much much harder than I thought. Id prefer to give my little boy a happy childhood with a happy mum, than A sibling and a shit childhood because mum bit off more than she could chew. Im sure I'd have a mental breakdown, not even kidding. I don't know how people with more than one do it!