If the military will protect the civilian population from zombies, then there should be no less than 47 people per 1000 civilians. But it's better to take the defense into your own hands, suggest the Brazilian mathematicians who modeled the zombie apocalypse.
A group of scientists from the University of Minas Geiras in Brazil posted an article “Modeling our survival in a zombie apocalypse” online.
The dynamic model created by scientists suggests that in the event of a mass zombie uprising, civilians will be protected by the military. The ratio of the number of armed soldiers to civilians turned out to be a key factor determining the resilience of the human community forced to fight the undead. If there are fewer than 47 troops per 1,000 civilians, survival is at stake. If a zombie apocalypse happens today, only North Korea will answer the challenge - it has just 4.7% of its population in the armed forces. Even taking into account the employees of the FSO and private security companies and without deducting the service personnel of the security forces, the percentage of armed and trained people in Russia barely reaches 3.9, analysts of the Seledka Nad Shuba Telegram channel note.
As a basis, scientists took a scenario in which the first zombie appears in a densely populated metropolis. Upon contact with him, healthy people themselves turn into zombies, their number is growing rapidly, and the situation soon becomes apocalyptic. Scientists relied mainly on cinematic material; according to these sources, humanity is quickly outnumbered and forced to live in isolated groups in fortified settlements.
The following assumptions were made: when meeting a zombie, a peaceful person kills a zombie, a zombie kills a person, or a person turns into a zombie (for simplicity, scientists have made these scenarios mutually exclusive). When meeting with a military zombie dies, because the military has weapons and special training. Changing the probabilities of different outcomes, scientists have compiled models describing four situations: 1) the population is protected by a Strong Army 2) the population is protected by a Very Strong Army 3) the population is protecting itself (Resistance), but at the same time people are more prepared and survive more often than in previous scenarios … In the fourth scenario, the survivability of the zombies was increased. This model does not promise anything good: strong zombies kill many people, but they zombify a little; as a result, only the military remain alive. The "Resistance" variant worked best of all:in this case, the zombies had no chance from the start.