It's just math, not sure it has a name, but I would call it a utilization study. You need to know how long it takes to repair a broken car to fully solve this. You also need to know what that 10% chance of breakdown means. Is it 10% per day, per operating hour? Those are big differences. You start by figuring out how much time the car can actually drive. Let's say the a broken car is out for 10 hours. For every five hours of driving time, you have four hours of charging and 10 hours x 0.1 = 1 hour of maintenance. Total five hours driving, five hours down. That implies two cars, but because your system is so small, you will have to have extra cars available. One car out of service is a big deal it takes ten hours to fix and you only have five hours of run time on the second car. I would run a Monte Carlo on the system to see how many cars will give you 99.9% uptime (or whatever your target it).