Scooters by Serial Number

A rideshare company stamps its rental scooters with consecutive serial numbers $1, 2, \dots, N$ and guards the fleet size $N$ closely. Walking through town, an analyst spots five scooters — a random sample without replacement — bearing serials $12, 33, 47, 68,$ and $75$. Using the minimum-variance unbiased estimator for this setting, what should she estimate $N$ to be?