When should I use 多少 or 几?
In English, we use &quot;how many&quot; for countable nouns and &quot;how much&quot; for uncountable nouns. But in Chinese, there's no difference between countable and uncountable when it comes to measure or describe the quantity of something.
We say &quot;how many men&quot; and &quot;how much money&quot; ; both &quot;how many&quot; and &quot;how much&quot; in Chinese is「多少」. &quot;How many men&quot; = 「多少人」; &quot;How much money&quot; = 「多少錢」

「幾」 (adj): a few ;  several - Example: 幾個人一齊做 ( a few people do it together)

「幾」 (adv):  quite; considerably - Example: 「幾好」( quite good)，「幾快」 ( quite fast)

「幾」 (adv): how; how many / how much ; what


Examples:
「幾多?」(how many? / how much?)
「幾時?」(what time? = when?)
「(有)幾好?」(how good?)
「(有)幾快?」 (how fast?)

「多少」(how many) is a phrase used to ask or talk about an amount ;「幾」 is mainly an adverb (how / what).  For example: 「幾」(how)+「多」(many) = 「幾多」(how many) ; 「幾」(what)+ 「時」(time) = 「幾時」(what time / when)
「多少?」and 「幾多?」only mean &quot; how many?&quot; but 「幾」has other usages.

When should I use 多少 or 几?

When asking [how many of something] or [how much of something] you can use  either[多少+(something)]or [幾多+(something)], for example: 多少人 or 幾多人
You can omit 多 in 幾多 in some cases when you want to use a more classical style of speech. Which means you can use: &quot;幾人? &quot; instead of &quot;幾多人?&quot;
Is the difference 多少 and 几 simply &quot;how much&quot; versus &quot;how many&quot;?  

The different usages of 多少 and 几 have been explained in the body of this post