"... that mirror." supposed to have the same meaning
Yes, same translationu, but not same meaning in English.
Yourself=berarti "kamu" cuma sendiri
Yourselves=berati "kamu" lebih dari satu
With kalian, it's "yourselves, "because kalian is plural. Kamu is singular. Even if they are both translated by "you" in English.
Apa beda yourself sama yourselves?
No. There is a singular "you" and a plural "you". If "you is alone, it's yourself, if it's a plural you like "kalian", it's yourselves.
yourself untuk satu orang/tunggal, yourselves untuk lebih dr satu org/jamak