几 or 多少

Both 几 and 多少 ask 'how many' or 'how much', but each fits its own frames.

• 几 fills a fixed slot with a small, expected range: the hour (几点), the weekday (星期几), the month and date (几月几号), the school year (几年级), the floor (几楼) and an order with 第 (第几次, 'which time').
• 多少 asks for a larger or open amount: prices (多少钱), phone, room and flight numbers (…是多少？) and nouns without a measure word (多少学生, 多少水).

Tip: 几 never goes straight before an ordinary noun. It needs a measure word (几个学生) or one of the fixed frames above, so you cannot say 几学生. 多少 can go straight before a noun or stand alone at the end of the question.