Do we say and write 21 / 31 / 41 item or itemS?
Solution 1:
In English, the singular is used for one thing, and the plural is generally used for anything else. This includes more than one (any number), as well as zero.
Solution 2:
Plural means more than 1. (Well, something like that. You can also have 0 items, which is less than 1, but still given the plural form.)
If you have 21 of something, then you have more than 1 of it.
The plural form of item is items.
So the correct way is 21 items.