What is the difference between “yourself” and “yourselves”? and when to use them?
Similarly say for “him” , “her” and others.

Use them when the action of the verb reflects back on the person of the subject:

You shot yourself.
You all injured yourselves.
He hurt himself.
She looked at herself.

You misunderstood. I want to know the difference between yourself and yourselves.
Similarly himself and himselves, herself and herselves and so on.

you is singular and plural, so you can say yourself (singular) or yourselves (plural)

He / she / It is only singular so you can only say himself / herself / itself - never himselves.

Them is only plural so it is only ever themselves - never themself.

That’s the difference - one is English (himself) and the other isn’t (himselves).

myself /ourselves
yourself / yourselves
himself / themselves
herself / themselves
itself / themselves

‘himselves’ and ‘herselves’ are not correct grammar.