Especially with those possessive pronouns, like 'hers' and 'theirs' ending in S already, you might instinctively add the apostrophe. But it is wrong.

'Hers' is a third-person possessive pronoun that we use to talk about something that belongs to a woman, girl, or female animal.

Sometimes, especially in written texts, 'hers' is used to refer to a generic person without considering that person's gender. Many prefer to use 'his or hers' or 'theirs' as an alternative.

Sometimes in formal and written English, 'hers' can be used to refer to a country, nation, car, machine, or ship.