"Somebody lost t h e i r keys." A single person. "They/Them pronouns can't refer to a single person."; Even when referring to somebody who identifies as male or female, you often swap out their pronouns with 'they/them/their', too. "Is that t h e i r dog?" A single person. "They wrote that" is far more logical than "He or she wrote that". A single person. Gender neutral pronouns are logically used in every day life, no matter how much you try to deny it. When you don't know somebody's gender online, you almost always use "they/them/their" when referring to them; a single person.