amount, worth (as in "two days' worth"), enough (for)
Number + Counter +
Attaching the suffix
Number + Counter +
Used as a standalone phrase to express the amount or worth of the specified quantity.
一週間
I bought one week's worth in bulk.
Number + Counter +
Used when the amount or worth directly modifies another noun, indicating 'a noun for [quantity]' or '[quantity] worth of a noun'.
三人
Tickets for three people
二日
Two days' worth of medicine
This construction is incredibly common with counters for time (such as days, weeks, or months) and counters for people, especially when discussing food portions, expenses, or supplies.
Noun +
Attaching
Expresses a portion, share, or specific calculated amount related to the noun it attaches to.
増加
Calculate the amount of increase.
不足
I will pay the shortage amount later.