你家有几口人？

You use this when you want to get to know someone better. It usually follows a question about their name or where they are from. To answer, you just swap 几 (jǐ) for a number. For example, 我家有三口人 (My family has three people). It is like a verbal Lego set; just swap the pieces and you are good to go.

Use this during small talk with new friends or colleagues. It is a great icebreaker at a dinner party. You can also use it when talking to a language partner. It shows you are interested in their personal life without being too nosy. It is the 'safe' personal question in Chinese culture.

Do not use this in a strictly professional business negotiation. It might feel a bit too personal if you are discussing a million-dollar contract. Also, avoid it if you know someone has a sensitive family situation. If you are at a funeral, maybe skip the headcount. Use common sense, and you will be fine.

In China, family is the center of the universe. Historically, large families were a sign of prosperity and luck. Even though the 'One Child Policy' changed family sizes for a while, the interest in family remains huge. Using the word 'mouth' to count people reflects China's deep history as an agrarian society. More mouths meant more hands to work the fields, but also more people to feed.

You might hear 你家有几个人? (Nǐ jiā yǒu jǐ gè rén?). This uses the general measure word 个 (gè) instead of 口 (kǒu). It is perfectly fine and very common in Southern China. If you want to be extra polite to an elder, you could say 您家有几口人? (Nín...). It is a tiny change that shows a lot of respect.