REQUIRED TO USE:
This is a PATCH not just a mod, therefore you need to download the Mod Patch Framework mod. To install this patch, just place it in the Patches folder located in your mods folder after installing Mod Patch Framework.
You also need the original mod.

Credit:

Complete and total credit for the REQUIRED original mod by flauschtrud. Who was generous enough to grant me permission to upload a patch for their amazing work. I barely did anything. This patch does nothing by itself and requires the Boarding Stable Lot Trait (NPC Horses at Your Lot) linked via "original mod" in this paragraph.

Note #2: You may want to increase how many NPCs can visit a lot via MCCC but it is not necessary if only one sim and the intended mares visit the lot.

What this does:

-Makes 8 horses spawn at the boarding stable instead of 3.

-(Optional, choose your version) makes the spawned horses male so that it becomes a stud farm for your mares.

Usage:

Add the Boarding Stable lot trait to a generic type community lot.

Visit the lot in live mode with your sim (and optionally their mares desired to be bred.)

Up to 8 (unplayed household) stallions that exist in any of your worlds on the save can spawn on the lot. If you don't have enough existing, it will spawn less than 8 horses.

Versions:

Stud Farm Version - This version spawns up to 8 stallions.
Only More Horses Version - This just adds more horses to the boarding stable mod, the gender is mixed instead of just stallions.

Tip: You can add Champion Horse trait to horses that appear at the stud farm via MCCC by shift clicking the horse, cheat sim info, add trait, general gameplay, Champion Horse (it's the first one.)
Tip 2: I highly recommend the "no relationship needed for encourage to breed" mod I made, found here.

[PW] Stud Farm Lot Trait & More Horses (Boarding Stable Patch)