“They” can be used as both singular term, or plural, with the context of the sentence making it clear which is being used. The same is also true for “you” which can also be singular, “Joey, you forgot your bag!” or plural “Students, you must get your books by the end of the day.”
The issue here is that many people weren’t taught the singular form of “they” in school, though it has been used in English for centuries, so there are people who feel that it sounds awkward and strange due to a lack of familiarity.
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