The ability to have a second child is not always there for many people. Including me. Wish people would think about this before extolling the virtues of having more than 1 child.
It really, really, really is not reasonable to expect parents of two or more kids to refuse to talk about their reason for having another child, when asked, on the grounds that "This might upset people who can't have more than one child." Seriously.
I almost stopped at one but had a last minute baby, so have seen both sides of the issue.
Life is generally easier, less stressful and less expensive during the childrearing years if you stop at one. On the other hand, there is a worrying sense of "eggs in one basket" if you only have one child. I know a couple of people who lost an adult child later on. In one case, there was a sibling. It wasn't that one sibling "made up for" the death of another, but it meant that the parents still had a family to bring them joy, something to go on living for, and grandchildren. In the case of the other family, where there were no other children, it was like a big void had been left in their lives and nothing really to fill it. It was a very sad situation.
On the other hand, it's also a rare situation, so I wouldn't recommend having a second child purely for those reasons. My own reasons were mainly that "Wow, my daughter is growing up, I'll be an empty nester in just over 10 years at this rate... I'm not ready for that, I want to do the whole childrearing thing again!"