“几个”is used to ask the quantity less than 10 most of the time.“多少个”is used more than ten, larger numbers.two sentences. You can see the difference.
1.你家有几个人？ni3 jia1 you3 ji3 ge4 re2? How many people are there in your family?
2.你有多少个朋友？ni3 you2 duo1 shao3 ge4 peng2 you3? How many friends do you have?