The study didn’t give a potential number of deaths, but it is estimated that it would be between tens and hundreds of millions. A scenario like this would also imply a lot of planetary side effects, such as firestorms, an immediate and massive Earth temperature drop, and a huge impact on food production.
If people didn’t die due to the explosives driven by nuclear energy, they surely would have because of radiation, starvation, and the combination of cooling and enhanced UV rays. As you can imagine, no proper temperature means that people couldn’t be able to plant cereals, fruits, and vegetables, and the whole food industry wouldn’t exist anymore. Experts say that things like these could easily trigger a global nuclear famine, and we wouldn’t be prepared for this to actually happen.