I'm trying to find some wheels for my fabia but I don't what is going to be the limit on width with standard arches am lowering ? I've heard 8j will be at a push but I'm not sure

It's not all down to the width of the wheel it has a lot to do with the et you can get 9j wheels fit with a high et. Eg 9j wheel with a et of 35 would fit no problem

Ride height plays a big part too

And tyre size ..

And diameter (16" or 17")