Similar use case that actually happens: hurricanes. Entire large cities evacuate. The Miami and Houston metro areas are both over 5 million in population.

Forecasts give days of notice, so I guess an autonomous vehicle ride-hailing service could have a million extra cars drives themselves in overnight. But refueling them would be a challenge. It's already a challenge for normal cars today (without using more fuel by rearranging fleets of cars between cities).

Also, sometimes they change traffic flow for evacuations, like contraflow lane reversal (https://en.wikipedia.org/wiki/Contraflow_lane_reversal) or using the shoulders as extra lanes. I wonder how well autonomous vehicle software handles that.