If Days Gone feels a little pedestrian to you and you and you want an experience more akin to the DayZ movie, then the Massive mod may be right up your street. It takes the size of the game's already expensive hordes and increases them enormously, leading to hundreds upon hundreds of the undead all clamouring to gut you. The experience appears about as terrifying for the player as it is for their PC, as it stutters and struggles with the sheer number of zombies hot on the player's tail.
Day's Gone is an expansive zombie survival game that ends up pitting the player against hundreds of zombies at once in carefully designed set pieces, but with plenty of random elements to make the experience that bit different for each person who tackles it. The Massive Mod makes the game even more of a challenge by more than doubling the number of zombies in even the largest of hordes. The modder's quick demo shows a horde of more than 600 zombies, where it was initially in the high 200s.
Potentially there's no real limit to the number of zombies that can be spawnd in, but as the modder notes on the Nexus page, it gets a little ridiculous in some places, with zombies spawning on top of one another or simply not fitting through some of the game's smaller action sequences where space is more limited.
You'll need to install the mod and then start a new game to get it working, but if you want the ultimate in massive zombie survival, this mod will give it to you.