There used to be a spare, usually set up for the fastest driver. If said driver crashed on the first lap and the race was red flagged, technically he could jump into the spare for the restart. The regulations in force at the time stated that should any race be stopped within the first two laps, the start would be null and void, and a full restart over the original distance would take place. Think Belgium 1998. Most teams had damaged cars, some both cars, so at the restart they could only run the one spare.