Xavier said in issue after that hundreds may have died, unless the character could somehow restrain the effects the reality of a global EMP would shut off global power and transportation and kill hundreds of millions to billions as whole regions of the world as in the power plants, every car, truck and boat would be slagged leading to global chaos and famine.I'd say Fatal Attractions beats that out. He sent out an electromagnetic pulse that cut off most power on Earth. So that could've led to fatal car accidents and entire hospitals shutting down.
The only way the scene works with anything like our universes science is he was restraining the effects massively otherwise there would have been global chaos which is the problem guesstimating casualties from something.