Here's the formula. It's basically the same thing that you all learned in high school to model population, but in our case it can be used to build a system that'll scale the zombies resistance based on the ratio of humans to zombies.
multiple = (MaxMultiplication) * (1 - (1/(1+e^(x-positiveOffset))) + multipleOffset
Here's a graphical example:
https://gyazo.com/97...5619f987d2557a0
I made this example a bit OP for the sake to show how easy it is to balance it. In this example, when the Human/zombie ratio is 1/1, the zombies resistance is nearly cut in half. Not good.
Simply adjust the numbers in a graphic calculator like Desmos, and you can find a more balanced example:
https://gyazo.com/f5...1cde851521f55ae
In this example, when the ratio of humans/zombies is 1/1, the multiple is 1. The game is acting as it normally would.
When the ratio is 4 humans to 1 zombie, the zombies get a 1.5x buff to their resistance.
When the ratio is 0.5 (Or about 2 zombies to 1 human) the zombies have 82.5% of their normal resistance, the lowest possible point is 64.2% of the zombies original resistance, but this is when the ratio is is 1/90. (1 human to 90 zombies ).
1.5\left(1-\frac{1}{\left(1+e^{\left(\frac{1}{90}-0.3\right)}\right)}\right)+0
Copy and paste this into Desmos and try it yourself.