幾 [measure/noun] = a few; [tens digit] 幾 = -teen; 幾 [tens place] = an unfixed multiple of ten
觸 · Trigger
You know the rough size but not the exact count, and would rather leave the digit open than guess one.
序 · The move
1Find the digit you cannot pin down.Is the count genuinely open, not a number you simply have not said yet?
2Put 幾 where that digit would go — alone before a measure for under ten, after a fixed tens digit for the teens-and-up, before 十/百 for an open multiple.Does 幾 sit in the unknown slot and leave the known digits as they are?
3Keep the measure word, the same as with a fixed number.幾 + measure, never 幾 on the bare noun?
例 · Examples
1A：你吃了幾how many / a few (the open digit)個general measure word餃子dumpling？B：十幾個。
A: How many dumplings did you eat? B: A dozen or so.
Same character, the slot tells them apart. In a question 幾個 asks for the digit; in a statement 幾個 supplies it as an unfixed few. 十幾 can only mean a teen — there is no question left to ask once a tens digit is fixed in front.
幾 stands in the digit's own slot (十幾 = somewhere 11–19, the ones place empty). 多 trails a round number kept whole and adds an open amount past it (十多個 = ten and then some more, 一個多月 = a month and a bit). 幾 fills the slot; 多 follows the round figure.
✗ 幾餃子 → ✓ 幾個餃子
✗ 二十五幾 → ✓ 二十幾 (幾 takes the ones slot, so the slot must be empty)
✗ 十幾百 → ✓ 幾百 / 一百多 (幾 fills one open place, it does not stack onto a fixed multiple)
English says the approximation as a separate word (a few, a dozen or so, -teen as a suffix); the learner reaches for an adverb and forgets 幾 occupies the digit's slot itself, and still needs the measure word.