Best idea for this is changing your gamestages.xml to increase the amount of zombies you get per game stage.
We made it go up faster on our coop server with 10 people, and we regularly lose 2 to 3 people on horde nights even behind 4 to 8 layers of steel.
You can also increase the amount of block damage zombies do during horde nights, so we have it at 500% on horde nights, 50% during regular days.
You can also change your WHZombiesMin and max to increase how many zombies are in wandering hoards.
We have it set that all horde have a minimum of 60 zombies, to a max of 180.
We also updated our CPU and memory on the server box, so we run with 600 max zombies at once, uses about 20gb of memory.