Caporegime
I think 10 maybe? I generally replace tyres at 3mm.
They have 8mm when new. Most passenger car tyres do aside from semi slicks which often come with 6.5mm~ (sometimes less)
I usually replace at 2.5mm if I ever keep a car long enough to wear the tyres out... But sooner than that if they are rubbish and I can't wait to get them off.