Table:
text: 1967 British Grand Prix
Classification
Qualifying
Pos No_ Driver Constructor_ Laps Time_Retired Grid Points_
10 18 Guy Ligier Brabham-Repco 76 + 4 Laps 21 
Ret 19 Bob Anderson Brabham-Climax 67 Engine 17 
Ret 6 Graham Hill Lotus-Ford 64 Engine 2 
Ret 4 Mike Spence BRM 44 Ignition 11 
Ret 9 Dan Gurney Eagle-Weslake 34 Clutch 5 
Ret 22 Silvio Moser Cooper-ATS 29 Oil Pressure 20 
Ret 11 Jochen Rindt Cooper-Maserati 26 Engine 8 
Ret 3 Jackie Stewart BRM 20 Transmission 12 
Ret 10 Bruce McLaren Eagle-Weslake 14 Engine 10 
Ret 17 Jo Siffert Cooper-Maserati 10 Engine 18 
Ret 23 Jo Bonnier Cooper-Maserati 0 Engine 9 
DNS 16 Piers Courage BRM 0 Non Starter

Question: How many drivers did not finish the race because of an engine failure at the 1967 British Grand Prix?