Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Pos | No | Driver | Constructor | Laps | Time/Retired | Grid | Points |
| --- | --- | --- | --- | --- | --- | --- | --- |
| 1 | 5 | Jim Clark | Lotus-Ford | 80 | 1:59:25.6 | 1 | 9 |
| 2 | 2 | Denny Hulme | Brabham-Repco | 80 | + 12.8 | 4 | 6 |
| 3 | 8 | Chris Amon | Ferrari | 80 | + 16.6 | 6 | 4 |
| 4 | 1 | Jack Brabham | Brabham-Repco | 80 | + 21.8 | 3 | 3 |
| 5 | 12 | Pedro Rodriguez | Cooper-Maserati | 79 | + 1 Lap | 9 | 2 |
| 6 | 7 | John Surtees | Honda | 78 | + 2 Laps | 7 | 1 |
| 7 | 15 | Chris Irwin | BRM | 77 | + 3 Laps | 13 |  |
| 8 | 20 | David Hobbs | BRM | 77 | + 3 Laps | 14 |  |
| 9 | 14 | Alan Rees | Cooper-Maserati | 76 | + 4 Laps | 15 |  |
| 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:
what is the number of drivers who didn't finish the race due to engine trouble?
Answer:
6