I don't have the exact numbers on hand, but iirc from gamestage 12 there is just a little chance added to find a blunderbus (tier1 shotgun). The chance to find a blunderbus increases with higher gamestages. Later than from e.g. gamestage 25 there is little chance added to find a pump shotgun. Same procedure, it's just a little chance that increases with higher gamestages.
Anyway, there is never a guarantee to find a shotgun, no matter what gamestage you have. You still have to search for it. Go for shotgun messiah crates, weapon bags and safes. If you don't go out for looting and/or only loot low tier loot boxes it will take ages to find a shotgun (however there is still a chance to find one).
BloodMoonEnemyCount ist just how many zombies you will get AT ONCE. A bloodmoon contains some waves, that may contain 20, 100, 500 or even several 1000 zombies depending on your gamestage. That has nothing to do with BloodMoonEnemyCount. If the waves for a certain gamestage contains 500 zombies, an BMEC set to 20 you will only see 20 of them in parallel. If you kill some of them there will spawn new Zs, but never more than 20 at the same time. That "respawning" repeats until either bloodmoon is over at 4 o'clock in the morning (with default game settings) or you fought of all 500 zombies that are defined for that gamestage bloodmoon (before 4 o'clock).
In later games, bloodmoon waves contain so high amount of Z that you most probably can't kill all of them before time is over.