The stuff you're referring to is Art Bell nuttery.
It is an interesting numbers game and some game theory might be useful. It is self-evident that an upper atmosphere EMP blast/blasts would shut down the electrical grid. Food spoilage would occur rapidly and certain MSA’s would be very hard hit as their access to food and water is reliant on its being delivered from outside of the areas limits. 70MM Americans live in the top 10 metro stat areas. And this population also has the largest concentrations of at risk population. For this hypo, they are defined as anyone with resources that only could support them for a few days. Low cupboards, food and water stock, or anything of value to trade.
As rational actors these areas would rapidly descend into chaos (2-3 days). No reactive or proactive Law enforcement activity could prevent it.
But what would the ultimate body count be. Millions would be lost in the 1st month. 4-5 million. Mostly due to dehydration, exposure and starvation. Most people as rational actors in conflict, would just hunker down. Hoarding supplies and going into a survival mode.
Cities outside the affected areas would see a huge influx of refugees, and would begin to assimilate them or set up camp areas that could be effectively managed.
The real question is how long we are without power due to EMP. I actually don’t think it would take too long to restore power. EMP blasts primary impact operating electronics and sensitive unsheilded devices (cellphones, towers, PCs, Tvs your basic consumer electronics).
So I imagine a timeline (assuming total national power grid failure) where, 1month we lose 4-5million, 3 month 5-10, and at a year (which I consider highly unlikely unless there is directed govt action against the people) the population should stabilize with ~20MM lost.
You can research these numbers against other disaster areas. The multiplier effect should be considered based on the amount of the power grid that goes down. Once it exceeds 75% of the country it should be considered a total power grid failure.
The Chinese would be happy to move 300 million serfs for the NWO into the former U.S.