By law, how many days in a row can an employee be made to work 12 hours per day?

The short answer is, "all of them." Except as indicated below, the law does not restrict how many hours or how many days in a row an employee could be made to work (though an hourly employee must be paid for all hours worked, including overtime when applicable).

1) If you have a written contract or union  agreement which specifies hours or shifts, you can't be made to work more than the contract allows.

2) A very small number of professions (pilot; truck driver; doctor) have maximum hours limited by law for safety reasons.

But other than 1) and 2), an employee can be made to work 365 12-hour days a year.