You just add it in to the server.cfg you can also do this using remote control commands.
I copied these from
here.
first put in sv_cheats 1 otherwise none of this will work
For bigger hordes:
z_mega_mob_size 70 // large mob sizes
z_mob_spawn_max_size 50 // Max size of a mob
z_mob_spawn_min_size 50 // Min size of a mob
z_mob_spawn_max_interval_easy 1 // max interval of horde spawninga on easy
z_mob_spawn_max_interval_expert 1 // max interval of horde spawninga on expert
z_mob_spawn_max_interval_hard 1 // max interval of horde spawninga on hard
z_mob_spawn_max_interval_normal 1 // max interval of horde spawninga on normal
z_mob_spawn_min_interval_easy 1 // min interval of horde spawninga on easy
z_mob_spawn_min_interval_expert 1 // min interval of horde spawninga on expert
z_mob_spawn_min_interval_hard 1 // min interval of horde spawninga on hard
z_mob_spawn_min_interval_normal 1 // min interval of horde spawninga on normal
director_panic_forever 1 // makes the ai director constantly panic
director_force_panic_event // forces a panic event (Horde to come)
for infinite ammo:
sv_infinite_ammo 1