Jǐ几 and duōshǎo多少 are both used to ask the number/quantity of people or things. While Jǐ几 is usually used based on the assumption that the number of people or things is less than 10. For example,
nǐ jiā yǒu jǐkǒu rén
You family has how many people
‘How many people are there in your family?’
Jiàoshì lǐ yǒu duōshǎo rén？
Classroom in has how many people
‘How many people are there in the classroom?’
Tip: duōshǎo多少 is usually used to ask the height, weight, length, size of people and things, and it is also frequently used based on the assumption that the number is more than 10. However, Jǐ几 matches with date, such as ‘jǐdiǎn 几点 what time is it’ and ‘jǐhào 几号 what date is it‘.