几【jǐ】

几 asks "how many" when the speaker expects a small answer – roughly ten or fewer. A measure word always follows it.

Leaving the measure word out gives nǐ你yǒu有jī几gē ge哥哥, which is wrong. 几 behaves exactly like a number, and a number needs a measure word.

几 against 多少

几 expects a handful; 多少 puts no limit on the answer and takes no measure word.

A family, a class timetable, the hour of the day – these take 几 because the answer is small by nature. A price or a population takes 多少.

Fixed questions with 几

几点 asks the time and 几号 asks the date, because both answers are bounded.

几 as "a few"

In a statement rather than a question, 几 means "several".