some days; several days — an indeterminate span of a few days.
彼は何日か出張で不在です。
He will be away on a business trip for several days.
風邪で何日か会社を休んだ。
I took several days off work because of a cold.
もう少し待てば、何日かで連絡が来るだろう。
If you wait a bit longer, you'll probably hear back in a few days.