How many patients have died in a ambulance ride?

It is hard to answer that question. Almost every 15 seconds somewhere in the US there is an ambulance going out to rescue someone from an emergency.

A patient could clinically die (go into cardiac arrest) in the back of the ambulance, but they are not legally dead until they reach the emergency room where a physician can pronounce them dead. So it depends on your meaning. for clinical death, it is not that rare anymore to get pulses back with all the drugs and tools a paramedic has at their disposal. Nobody legally dies in the back of an ambulance.