What percentage of people that flip their car die?

Answer 1

You need to ask the question more specifically for a relevant answer.

The biggest factors to consider would be:

Seatbelts- people who are belted in when a vehicle assumes an unusual attitude (flip, roll, spin, crash etc.) tend to survive. People who are not belted in tend to get ejected from the vehicle or bounced around inside the vehicle and suffer severe trauma.

Speed of incident - Force is equal to Mass times Velocity times Velocity - F=MV squared. This assumes there is an impact involved. Flipping a vehicle without a final resulting impact would exert forces on the occupants but, if they are belted in, they should survive (think roller coaster).

Vehicle design - hardtop vs convertable or the back of a pickup.

Still, the number one factor in surviving any unusual circumstance in a vehicle is to be wearing a seatbelt.