Looking at rates of corpse decomposition could lead to an upper estimate of zombie/skeleton feasibility. A buried corpse (not interred in a tomb, but buried in the ground, either with or without a coffin) will hold together and be recognisable as a corpse for between one and eight years, before it could be described as a skeleton. After that the skeleton will have bone structural integrity for perhaps fifty to eighty years depending on soil acidity, ground movements, animal interference etc. You can fudge these figures higher for tomb interred/mummified/bog preserved or frozen bodies.
So for any population, the maximum amount of zombies possible would be in line with the mortality rate for lets say the last ~4 years, and the maximum number of skeletons ~65 years (the mortality rate for ~70 years minus the corpses that could viably be zombified)
So the questions are- what are the mortality rates for the different species and settlements? Also, does childhood mortality have an impact? And finally, do undead creatures continue to decompose once resurrected? I.e. does a zombie continue to rot and eventually turn into a skeleton, or do they stop decomposing at the point of resurrection?
To take this further, the crude death rate for the real world hovers somewhere between 7 and 10 deaths per 1000 populous per year. We can fudge that higher for a fantasy world with lesser medical care, constant warfare, poorer welfare and arguably shorter life expectancy. The more grim you want your world to be, the higher the crude death rate. I would reasonably cap the viable crude death rate of the entire Warhammer world to 30 per 1000 populous per year, and a similar birth rate to allow a state of constant repopulation.
So for the maximum number of reanimated zombies in the Warhammer world we're looking at approximately 4 years of deaths at 30 deaths per 1000 people, or 12 zombies per 100 living people. Meaning that in each civilisation with a necromancer or resurrection event, you can have between 0-12% of the population figure in zombies. Likewise, reanimated skeletons would be a maximum of 0-195% of the population. A village could very realistically be overrun by and outnumbered by reanimated skeletons, less likely to happen with zombies.
Zombies = 0-12% of the current world population
Skeletons = 0-195% of the current world population.
Lower figures for jungle/rainforested areas, higher figures for desert, icy, peat bogs, religions who entomb or preserve their dead. Norse undead are unlikely if they burn their dead, as are races who eat their dead beastmen? Orcs?