The stalled current of the starter is determined only by the total resistance of the battery, leads, brushes and windings. Because the starter isn't rotating, there's no back-EMF to reduce the current.
If you absolutely must know what the current might be, you can use a multimeter to measure the total resistance of the starter and leads. Current =12/(resistance). It will be a big number, but the actual current won't be more than that.
My starter set-up is limited by lead resistance rather than the battery performance and I start my big engines (1.20's) by hand. Once the knack is learned, I can get 'em to go, usually on the first flick.