几 and 多 can combine to form a special word 几多 that can be used as an interrogative pronoun or adverb. As a pronoun, its meaning is equivalent to “how many/much 多少”, but it’s far less often used than 多少.
几多 how many/much
From a practical point of view, 几多 is not so useful as 多少. 几多 is used in some dialects or sometimes in some poems, and also it can be used to modify abstract nouns such as 几多风雨几多春秋. 多少 is very useful and can be used anywhere to mean “how many/much”.
Examples:
If you ask me how much my sorrow has increased, just see the over brimming river flowing east! 问君能有几多愁, 恰似一江春水向东流。
How much does this sack of rice weigh? 这袋米有几多重？
How many people? 几多人？
几多 what an extent
几多 can be also used an adverb to express an exclamation with the meaning “what an extent”.
Examples:
这孩子几多聪明！What a clever kid!
这小女孩几多懂事！How sensible this little girl is!