As you can see from my answer there, it depends. If you ask something that might generate a (very) long list, or even potentially infinite, then it's a bad question. That's because the SE model is geared towards more definite answers.
However not all list questions are bad, some have very definite answers even if technically it's a list question. For example, if you ask "how many languages possess this unusual phenomenon?", it would generate a very small set of answers, because the trait you asked about is unusual or rare.
Certainly if you asked, "how many languages have vowel sounds", then that would be a very bad list question.
If you're unsure about a question you want to ask, you can post on Meta asking for support. I'm sure other users will help you determine if your question is a good fit or not, and maybe even help you with the wording if necessary (which makes a world of difference).