expression
some days; several days
Refers to an unspecified number of days, typically a small number. Often used when the exact count is unknown or not important.
幾日か休みを取って旅行に行きたい。
I want to take a few days off and go on a trip.
この仕事は幾日かかかるだろう。
This work will probably take several days.
数日 is more formal and commonly used in writing, while 幾日か is slightly more conversational and can imply a smaller, vaguer number of days.
Combination of 幾 (いく, 'how many') + 日 (にち, 'day') + か (question/indefinite particle). Literally 'some number of days'.