Translation:What is your phone number?
Maybe he/she hasn't been active with the language for quite some time, or maybe wants/needs to improve its skills in the language.
Also, it's great to have several Chinese speakers here, be it natives or not, as they can provide great help, as seen here.
I'm fluent in Swedish, and I've "hammered" it quite a bit here, but I also know there's still plenty for me to learn, and I've learned a bit of new things already! So it doesn't really matter if you can speak a language just a little bit, or really well, there's still always something new to learn.
The better you get at a language, the more help you can provide to new learners too, if that would ever be of interest to you, so it's a win-win anyways! :)
Yep, it is the "what" used for quantities/numbers. Textbooks say it's translated as "how much" or "how many", but as you can see in the sentence here, it has more uses. It's often used when you know the answer will be more than one digit/cipher.
If the expected answer is a number less than 10, then 几 jǐ is used in place of 多少 duōshǎo.
This is what I've learned:
- 你的电话（号码）（是）几号？ (This version is common without "是", but it also occurs with it.)
Native Mandarin Chinese speakers have their individual opinions about these, because China is a big place, not to mention the outlying Chinese-speaking countries and regions, and this is a colloquial sentence influenced by local dialects and by quirks of history.
1 is somewhat more northern, but common in a lot of China, and perhaps most common overall. 2 is perhaps half as common as 1, but it's more southern, and common in Singapore, Taiwan, and Malaysia (and perhaps other places, e.g. the southern mainland or some of it), where people will often swear they've never heard 1 (and likewise, proponents of 1 will often swear they've never heard 2). In each of these two cases, proponents of one version will say why the other isn't logical, and none of their reasons is really very convincing. It seems the usage of one or the other is really about history, habit, and experience.
3 is fairly common, but perhaps not as common as 1. It feels unnatural to some speakers (particularly to many who like 1) but not to others (those who like 2 are usually fine with it, as are some who like 1). Those who don't like it will even go so far as to say it sounds like you don't really know what a telephone number is, or even what a telephone is.
Where English has a greater influence, 3 seems like the more correct or formal structure to some speakers. That's not to say that 3 doesn't have inherent support by the rules of Chinese itself, but it's not necessarily the go-to phrasing historically in broad swathes of the Mandarin-speaking world, and some native speakers say they never hear anyone use it. But many native Mandarin speakers will swear that it's the only truly correct option.
There are still other ways to ask this question, including shortening the above sentences, but I would stick with those three choices to begin with.
Here are some Youtube videos teaching the "多少" version (which is the most common version on Youtube):
I think the answer is yes. If you expect that the answer is some numbers you can generally use 多少 to ask the question, such as room number, ID number, etc. I am not sure if there are exceptions but I can't think of any. You can also use 几号(jǐhào) in such situation: 你的电话号码是几号？.