In duolingo, it rarely “tells” you the definitions of things, and leaves you to suss it out from context.

I’ve also had a similar experience when learning Japanese with it.

多少 roughly translates to either “how many” or “how much,” but it can also be used when asking for almost any kind of numerical answer. Your understanding of 他的电话号码是多少？ is almost correct. It means “what is his phone number?” If you want to ask someone how many apples they have, you would say 你有多少颗苹果？ or 你有几颗苹果？. (几 is “how many.”) Another few example sentences:

• 你有多少根香蕉？(you have how many bananas?) (How many bananas do you have?)
• 你的体重多少公斤？ (your body weight how many kilograms?) (How much do you weigh?)
• 你还有多少面包？ (you still have how much bread?) (How much bread do you have left?)
• 你刚刚吃了多少薯片？ (you just ate how many potato chip?) (How many potato chips did you just eat?)
• 你家里有多少本书？ (you home have how many [classifier for books] book?) (How many books do you have at home?)
• 你的身份证号码多少？ (your ID card number how many?) (What is your ID number (social security number)?)
• 你一个月的收入多少？ (you one month income how much?) (What is your monthly income?)
• 你手机的容量多少？ (your phone capacity how much?) (How much disk space does your phone have?)

It’s ok if you don’t understand some of the words I used. Only after I typed those sentence out did I realize that some of them might be too difficult, so don’t be intimidated by them.