I have four kids all under 6. I know. Insane.
The way I see it is much the same as others have said. Generally, more than one child IS harder work. Not always but usually.
However, it also depends on the individual child/children. Mine for example are all extremely strong willed, high energy, stubborn, defiant, messy, loud etc. I know many other kids are like this. But, I also see their friends and children of my friends who aren't like this and seem to be "easier" children. I'm utterly exhausted and stressed most days. Quite often, I find other parents of 1 or 2 children struggle to relate to certain aspects of my day or how my children have been acting etc.
Also, things tend to seem much easier when 1 or 2 children are removed from the equation (in school/childcare or at grandparents etc) but it depends which ones 😂
But at the end of the day you aren't any less of a parent and trust me, you aren't missing out. If it makes you feel any better, I've noticed this summer more than ever how I'm slightly envious of all the social media posts I see from all the mums I know with one child having nice days out and trips and lovely calm activities with them. Going out with multiple young children is a mission and tbh rarely that enjoyable.
The grass is always greener 😂