Herself is correct and a reflexive pronoun in English. Herselves is incorrect and should not be used in English. Her self is used in a different context.
She needs to bake the cake herself.
Herself is the reflexive pronoun for the pronoun “she”. We use a pronoun in place of a noun when we don’t want to state the noun. A reflexive pronoun is when the pronoun is both the subject and the object of the sentence. This happens for example when the person(or people) is both the doer and receiver of an action.
She has to cut the grass herself. We had no gardener.
Herselves is not correct in English. The pronoun “she” is always singular in English.
“Her self” is not used as a reflexive pronoun but “her self” can be used in another context. “Self” is a noun that means a person’s essential being. In this case, you can use “her self”
Her self has many layers.