How many 12 hour days can you employer work you

Absent certain professions (i.e. truck drivers, airline pilots, certain medical personnel, etc.), there is no legal limit to how many days in a row an employee can be scheduled to work. That having been said, if you are a non-exempt employee (typically hourly) then you are subject to overtime for any hours that you may work over 40. The above is true unless the employee's treatment constitutes some form of legally actionable discrimintion or thios action violates company policy or any applicable union agreement or employment contract.

They can ask you to work 365 12-hour days a year without giving you any time off: there is no law restricting how many hours per day, or how many days per year or days in a row, an employee can be made to work. Anyone not willing to do this can seek other employment--"employment at will" works both ways: your employer can terminate you at any time, if they don't like or want you, but conversely, you can quit at any time if you don't like or want the job. If you are a non-exempt hourly employee, however, you must be paid for all hours worked, of course, and also overtime when you work more than 40 hours in a workweek.
If you have a written employment contract, however, which sets or specifies your hours (including a union or collective bargaining contract), they can't make you work more hours than are in the contract.