I use these guys. I use their static cling numbers, very light plastic or vinyl of some kind. You put them on wet and squeegee the water or windex out from under them. I put a bit of scotch tape on the leading edge to ensure they don't lift and fly off.
GoChecker.com Custom Static Cling Race Number Decals, Vinyl, Magnetic
I agree whole heartedly with Capt Carrera - hitting someone else's detached magnetic number meatball can not only ruin your paint, it can break or spider web fiberglass parts (front bumpers, etc.) if you hit one at speed. I've seen it happen. Some organizations prohibit them, for good reason.