Asking “how many”: 几, 多少, 多 + adj.
【一04】【一46】疑问代词 多、多少、几
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
几 (jǐ) is used when you expect a small number (usually under 10), and for dates and clock time. It is always followed by a measure word: 你家有几口人？ 现在几点？
多少 (duōshao) is for any or large numbers, and the measure word is optional: 你们班有多少（个）学生？ It's also the word for prices (多少钱？) and numbers (手机号是多少？).
多 + adjective asks about degree: 多大 (how old / how big). 你多大？ = “How old are you?” (between adults). To a child, ask 你几岁？
Age needs no verb: 我二十岁 (“me twenty years”).
Examples
你家有几口人？
Nǐ jiā yǒu jǐ kǒu rén?How many people are in your family?
你们班有多少个学生？
Nǐmen bān yǒu duōshao ge xuésheng?How many students are in your class?
你哥哥多大？
Nǐ gēge duō dà?How old is your older brother?
小朋友，你几岁？
Xiǎopéngyou, nǐ jǐ suì?How old are you, little one?
你的手机号是多少？
Nǐ de shǒujī hào shì duōshao?What's your phone number?
Common mistakes
✗ 你家有几人？ → ✓ 你家有几口人？ (几 needs a measure word).
✗ 我是二十岁 → ✓ 我二十岁 (no 是 for age in positive sentences).
✗ 你几大？ → ✓ 你多大？