[dkfmn]

>you only have one credit-card

nothing inherently wrong with that. You won't have maximum access to credit and multiple bank relationships but those consequences may not be relevant to you.

> you make sure you don't overspend

That's great, but not related to the number of credit cards someone has.

> never paid attention to how many points you accumulate.

Some people obsess with points but there's a difference between not accumulating optimally and not accumulating at all. It's incredibly simple to get a 1-2% discount on all money you spend by using credit cards. But, again, that opportunity cost may not be relevant to you.