Assuming stock wheel thickness (lug nut to mating surface distance), stock studs, and stock lug nuts you have about 13-14mm extra stud to work with before you run out of threads on the stud completely. This isn't considered very safe to have no threads left on the stud. Also the wheel will no longer be resting on the hub (won't be centered) and will be using the studs only to hold the weight of the car.
The thickest spacer you can go and still be sort of safe is 8mm.
You can go with 20mm (or is it 25mm?) spacers that incorporate their own set of studs. These are fine and they can go from 20-25mm all the way up to 50mm or more.
Rubbing in the rear happens with stock wheel/tires around 18mm or a little more and rubbing in the front is unlikely ever.