Tire, tire diameter and section width mainly. For 33x12.5x15 4in BS is a safe bet regardless of the tire provided fenders are trimmed and axle positioned properly.

If you want to run narrower (like a 10.5), you can go up to 4.5BS.

The smaller the backspace, the more the tires stick out. If you like that look, then just go with 4 or less. If the fenders have to cover the tires to pass inspection, you'll want to proceed a bit more carefully.