https://en.wikipedia.org/wiki/24_Hours_of_Le_Mans#Drivers
Initially, there were no rules on the number of car drivers or how long they could drive. Although almost all teams used two drivers in the early decades, some Le Mans drivers such as Pierre Levegh and Eddie Hall attempted to run the race solo, hoping to save time by not having to change drivers. This practice was later banned. Until the 1980s, there were teams in which only two drivers competed, but by the end of the decade, the rules were changed to stipulate that at least three drivers must drive each car.
By the 1990s, due to the speeds of the cars and the strain it puts on drivers, additional rules to reduce driver fatigue mandated that drivers could not drive for over 240 minutes over 6 hours and that no one driver could run for over 14 hours total. With careful management of driver stints, this makes it possible to complete the race with only two drivers (as Jeroen Bleekemolen and Cooper MacNeil did in 2014), although the vast majority of teams still continue to use three drivers.[16][17]
In 2017, the driving time rules were further changed. If necessary, officials may require a drive time limit of 80 minutes of consecutive time behind the wheel and a minimum 30-minute rest break. The rule applies only if the air temperature is at least 32 Celsius (89.6 F).[18]