drivers per team has evolved over time. Initially, teams of two were the norm. One driver rested while the other driver drove. There was also the 'Like Lindie, I'll go it alone" attitude for a solo attempt. Arguably the most famous person in this category was Pierre Levegh who almost won the race in 1952 driving alone. In the final hour, with a four lap lead, his car suffered engine failure.
Coming into the pits is not only for driver fatigue, but also car maintenance. Levegh (who was driving with a co-driver in the Mercedes team) was the driver involved in the crash in 1954 that killed spectators in the finishing stand (opposite the pits). It remains the most horrific crash in motor racing history: 83 spectators died.
These days, three is not only the norm for a team, it is a requirement.
Andrew H