"It depends"
You can get away with much more on an older vehicle vs. a newer one (just to pick a number...pre-70-ish, vs after).
I talked to the DMV officer several years back (12+) about replacing the body on my '73 CJ. He said the law wasn't clear, but in his opinion, as long as I kept 1 sq. foot around the VIN tag that is rivited to the body (on the firewall in the engine box for the CJ) then it would be OK. Dunno what applies nowadays. Probably can't get by with near as much.