If you are sure that the number is less than 10, you can use either 几 or 多少 in your question. Otherwise, you can only use 多少.

In question sentence they translate to how many or how much, and you have to add measure word behind 几, 几 means several, it must be less then 10.

e.g.

你们来了几个人？你们班有多少人？

苹果几块钱一斤？黄斤多少钱一斤？

（But it seems we also say like this:你们班有几个人？黄斤几块钱一斤？ although we know it must more then 10. I don't know why!）

You can only use 几 to discribe several in the indicative mood, in this case you can't use 多少 instead of 几

e.g.

我买了几个苹果。（I bought several apples ）(我买了多少苹果 is incorrect, is sounds like how many apples did you bought?)

When you want to ask someone's number, you have to use 多少

你的手机号码是多少？

Difference between 几 多少

Can you explain me the difference between 几 and 多少？
When I may use it? With examples please!!

几 is a bit more formal and more respectful than 多少 in certain context.  For example, "How old are you?" would be 你几岁呀?  Using the other term though certainly understood, sounds very disrespectful because of the dimunitive term 少 (unless spoken to someone of inferior status than you are).  你多大? is better as 大 denotes respect.  In Chinese culture, the older person deserves more respect unlike the west.

"How much money do you have?" In this case, both terms, 你有多少钱 or 你有几多钱 is perfectly acceptable.