You are asking a quantity and have to choose whether the answer is a small count or an open amount.
序 · The move
1Judge the expected answer: a small number you could count off, or any amount including a large or unknown one.Would the answer plausibly sit under ten?
2Small count → 幾, and fit a measure word straight after it. Open amount → 多少, measure word optional.Does 幾 have its measure word? Is 多少 left to stand alone?
3Set the noun behind the measure word for 幾; behind 多少 directly (or with a measure) for 多少.Did you avoid bolting a measure word onto 多少 錢 / 多少 水 where none is wanted?
例 · Examples
1從臺北到臺中，坐火車要幾how many (small count)個measure word小時hour？啤酒一罐one can多少how much (open amount)錢money？
From Taipei to Taichung, how many hours by train? How much is a can of beer?
幾 expects a small countable answer and demands a measure word (幾個人); 多少 expects any amount and takes none for mass things (多少錢). Pick by whether you expect a small count or an open quantity.
數+量 is the answer frame — a fixed number plus measure plus noun (三個小時); 幾 / 多少 are the question that leaves the number blank. 幾 keeps the measure slot, 多少 drops it.
✗ 幾錢？ → ✓ 多少錢？（price is an open amount, not a small count）
✗ 多少個小時？（heavy where a small count is meant）→ ✓ 幾個小時？
✗ 幾人？（measure word dropped）→ ✓ 幾個人？
English uses one phrase, "how many / how much," split only by countable vs mass; learners reach for 多少 everywhere and forget 幾 must carry a measure word.