How can a employer tell me that I can’t work 12 hours or 16 hours but lets other employees do it?

Employers are, except as indicated below, allowed to be unfair or inequitable, and to treat some employees differently than others. So as a general matter, an employer could let John Doe work more hours while denying James Doe the chance to do the same.
What employers cannot do is discriminate on the basis of a specifically protected characteristic, the main ones of which are race, religion, age over 40, sex, and disabilty--so while an employer could let John Doe work more than James Doe, it could not let John Doe work more than Jill Doe if the only reason he is getting more hours is because he is male and she is female. Though the employer could John work more hours than Jill if there was non-discriminatory reason for doing so, such as that John has seniority over Jill or better education/training.
If you feel that the reason you are being denied extra hours is discrimination due to a protected characteristic like your race, religion, sex, age over 40, or disabilty, you may wish to contact the Equal Employment Opportunity Commission EEOC or your state's equal/civil rights agency to discuss filing a complaint.