Chinese has two ways to ask "how many". 几 (jǐ) is for small numbers and always needs a measure word. 多少 (duōshao) is for any number, large or small, and is also how you ask about prices and phone numbers.
几 + measure word + noun.
Use 几 when you expect the answer to be a small number, usually under ten: people in a family, children, things you want to buy. 几 must always have a measure word after it, such as 个, 本 or 口. To answer, put the number where 几 was. There is no 吗 at the end, because 几 already makes it a question.
多少 + (measure word) + noun.
Use 多少 when the number could be large, or when you have no idea what it will be. With 多少 the measure word is optional: 多少学生 and 多少个学生 are both correct. 多少 is also the word for prices and for numbers such as phone numbers.
Some everyday questions always use 几, because the answer is a small number: the hour (几点), the day of the week (星期几), the month and date (几月几号) and a child's age (几岁).
几岁 is for children. To ask an adult's age, say 你多大了 instead.
The most common mistake is to leave out the measure word after 几. 你有几书 is wrong; say 你有几本书. If you are not sure which measure word to use, 多少 is the safer choice, because it works without one.
In a sentence that is not a question, 几 + measure word means "a few" or "several".
Practise this grammar point on paper: 20 exercises using only HSK 1 words, with an answer key on a separate page. Free to print and copy for your class.
Every example on this page uses only words up to HSK 1. Paste your own sentences into the level checker to see which words, if any, are above your level.