When you search for wheels and tyres for superb3 it says that 9" wide is the biggest you can fit. I want to get a 20" 9,5" wide. Does anyone know if this will fit?

Depends upon the offset. If the outer diameter is the same (inc tyre) then where the rims sit at all points on the suspension arc will be key. What's the overall width inc tyre of your chosen tyre pattern and how much room is there in the wheel arch before you hit anything?

Haven't looked how much space there is on the inside, but I can move the wheel at least 2 cm out without being illegall. So a low et should work.