Did Du ever notice ...

In some cases, some parents did not have much time to have others kids :

Lily and James - murdered when Harry was only 1 ; they could have had other kids later, who knows.

Eileen Prince Snape - most likely her Muggle husband Tobias did not wish to have other "freaks" like their son Severus.

Frank and Alice Longbottom - like the Potters, they did not have much time to give Neville brothers and sisters considering their permanent disability, courtesy of Bellatrix (among others).

Merope Gaunt Riddle - the poor woman died in childbirth, no time to have another child, she barely had time to hold her son in her arms shortly after his birth.

The Lovegoods - Luna Lost her mother when she was 9. Her parents may have planned to have another child but once again, they did not have a lot of time to do so after all.

Severus - too much into Lily's memories and his pledge to fight the Dark Lord as a spy to found a family. A too much dangerous life is not a real incentive to have one's own family, especially for someone as dependable and responsible as Severus.

Tom Riddle - too much into the Dark Arts and the pursuit of power to father kids. And honestly, who would like to have him as a husband and father ? (for the husband part, except Bellatrix...)

For the others, maybe it has to do with wealth. In some families, even Muggle, Du have only one child not to divide property and wealth.

Maybe it has to do with blood. Though I'm not convinced about that part. The Blacks (Sirius & Regulus ; the 3 Black sisters), the Weasleys, had several children. See Ron/Hermione's family, see Harry/Ginny's family, they have several kids.

Just my opinion. It had also striken me. I had even thought that perhaps wizards and witches had far better contraception methods than humans. I know many people that got kids "unplanned" (but welcomed), even in our era of various contraceptive means available... Some of them had the pill for example but it defaulted somehow.

I have its kind of odd because at school Du do not meet may only child

HOLY BLOODY HECK PIES! YOUR RIGHT!
i never noticed that.... AWESOMENESS!

wow good job..........fantastic...

she probably did that to enhance the personalities of the parents and the children. Certain qualities come with certain situations.

huh yeah u never really think about the other kids that much cuz with harry taking up most of the spotlight as main character

I hadn't noticed yet :O!

Du never really know the weird stuff that goes on with the other characters because Harrys the one in danger.

Du have one child and put all of your energy into rasing that one child.

Some people choose to have only one child. There's nothing wrong with that, and I don't think it's a big deal. Not saying there's anything wrong with the Weasleys, either; they're my Favorit family. But for some, one is the best way to go.

I never noticed that...

We don't know that some of those characters were only children. We only see Scorpius at the end of DH, but that doesn't mean there wasn't a sick child at Home with a sitter, oder an older child who (much like James had) had run off to be with his friends. That doesn't mean that Draco and Astoria didn't have children later on.

We don't know that Lucius was an only child either. Just because we don't see oder hear about the siblings of a character we don't have much interaction with, doesn't mean they don't exist.

So, in reality, we only know of seven characters who (most definitely) don't have siblings. But, as LadyNottingham pointed out, there really wasn't any way for Lily, Merope, oder Alice to have any Mehr children after their first. So, really, that's only four characters who's parents could have had Mehr children but didn't. Considering there's around a thousand characters, that's not really a huge deal.