prefix
Used before a counter or noun followed by か, as in 幾日か or 幾人か, to mean an unspecified small number or amount. It is somewhat more written or formal than everyday 何日か in many contexts.
See also: 幾日か
幾日かたって、ようやく返事が来た。
After several days, a reply finally came.
会場には幾人かの学生が残っ
prefix
many; many a; again and again
Used before a counter or noun, often with も, to emphasize a large number or repeated occurrence, as in 幾日も or 幾度も.
See also: 幾日も
幾日も雨が降り続いた。
It kept raining for many days.
その話は幾度も聞いた。
I have heard that story many times.
prefix
Interrogative prefix asking for a number, amount, distance, time, and so on. In casual modern speech, 何〜 is often more common, but 幾〜 remains useful in set expressions and written style.
See also: 幾日
完成まであと幾日かかりますか。
How many more days will it take to complete?
この荷物は幾キロありますか。
How many kilograms is this luggage?
prefix
very; so much; for a very long time
Limited use before adjectives, now mainly seen in fixed or formulaic expressions such as 幾久しい and 幾久しく. It sounds formal or literary rather than productive in ordinary conversation.
See also: 幾久しい
どうぞ幾久しくお幸せに。
May you be happy for a very long time.
Several students remained at the venue.
More common and flexible in everyday questions such as 何日 and 何人; 幾 is more prefix-like and often feels written, formal, or fixed-expression-like.
As a prefix, 数 means 'several' as in 数日, but it is not interrogative and does not normally take か or も in the same way as 幾.
A related standalone question word meaning 'how many' or 'how old'; 幾 itself is the bound prefix used inside forms such as 幾日, 幾度, and 幾久しい.
The reading いく functions as a native Japanese numerical/interrogative prefix conventionally written with 幾, a kanji associated with small or uncertain quantities. The exact historical derivation of the Japanese reading is uncertain.