Out of 7 billion, I would say that at the very least 50,000 would survive and obviously, there would be a new world order. (It's only logical) Anyway, anyone who can survive the apocalypse is resourceful. Sure, the start might be a little tough but humans will always swarm together, one way or another.
Barring worst case scenario, there should be plenty enough survivors but the variables are immense so who really knows what may happen. There are as many different potentials as there are people XD
@Ai chan Umm...... I am not an undead... But a non-undead-alive-corpse-living-body... Basically, my body and mind are alive, but my soul is dead... as with any form of discipline and diligence... Find the ground zero before the outbreak spread. Nuke them from orbit. Nuke them twice to make sure. Nuke them thrice to make double sure. Nuke them viermal because nuking things are fun. Then saturate the area with carpet bombing every hour every day for 33 months. If all is failed, send spaceships to 433 Eros and other NEO and tweak their orbit for earth collision to turn the surface into a sea of magma. Because why, because if we can't get the Earth, then no one can...
System? Hell no! You gotta do it raw. Like how them hunters did in the olden days. Pray to your gods you live or survive the virus (perhaps even mutate). Non-undying [living organism] + alive-corpse [artificial vessel] + living body [reanimated soul] = Frankenstein Got it, you're not a zombie, but THE ZOMBIE! Doc... we would be long dead from the bio-hazardous environment.