I play on a dedicated server and get 600 zombies a horde night. It usually lasts from 10 - 4am. I have had this since the day 7 horde.
In that case, your party had made it to gamestage 147 (or higher) by day 7. Which on 2-hour days and with diligent XP hunting might not be all that difficult. The GS147 horde is the first horde where the total possible number of zombies > 600. It is the "num" parameter that determines the total number of possible zombies in each wave. Add 'em up for the total possible zombies on horde night (absent a mod like mine to ensure the wave goes on forever).
<gamestage stage="147">
<spawn group="feralHordeStageGS133" num="207" maxAlive="47" duration="2" interval="20"/>
<spawn group="feralHordeStageGS140" num="207" maxAlive="47" duration="2" interval="20"/>
<spawn group="feralHordeStageGS147" num="207" maxAlive="47" duration="7"/>
</gamestage>
The full explanation is
here, but note that the maxAlive is
per player, so if you've got an 8-player horde party, then each wave would send the LESSER of these options all at once (spawning new Zs to keep the queue full as Zs are killed):
A) maxAlive x players = 47 x 8 = 376
B) "num" value = 207
C) Server max spawned zombies = 75
D) Server BloodMoonEnemyCount setting x players = ?? (default setting is 8, so this would be 64)
My guess is you got either 64 or 75 zombies at once, until the 207 "bucket" for each wave was killed. With 8 players doing the killing, I'm kind of surprised it would last until 4AM. The duration of the first two waves is 2 in-game hours, so 10 minutes of real time. If each player only kills 3 zombies per minute you'd easily wipe out a 207-zombie wave with time to spare.
My 2-player co-op game is past that GS and we finished the normal horde by 1AM or so. Note that the total size of the horde does not change with # of players. Only the # of zombies spawned in at once. My 2-player game and your 8-player game both would need to kill (at most) 621 zombies to complete the GS147 horde night.