An experimental submod for Astartes Overhaul (upon some further testing, it still works without it, but best for full experience) to increase the number of enemies that spawn in as well as having more bots such as full 10-man squads based on tabletop compositions. Credits go to Warhammer Workshop, Saint Marold, and the mad lads at the Increased Spawns and Larger Squad Sizes thread over at the Astartes discord (I'm using mixed values from their mods as a foundation).

NOTE: This is not balanced in any way. It was never intended to be balanced, and you should either not expect to win a mission with this mod or, reversely, breeze through it depending on the squad. This is strictly just for the fun of having large-scale battles with the NPCs, 40k-style. It is also meant to be played on Lethal. I am not sure how it'll operate in lower difficulties. This is mainly a personal mod that I wanted to share due to requests from several users. It was mainly meant for quick bursts of action to take screenshots with, it is possible that the game will spawn too much NPCs for it to handle and, depending on your system, probably crash. Also, I run my own personal set of tweaks with a couple of features from Astartes sprinkled in. There might be some bugs. Please report any that you experience.

I also highly recommend you grab Active AI to make enemies more responsive and aggressive. 

MAIN FEATURES:
-larger enemy count (idle groups and horde groups are more numerous)
-more ferocious and intense AI director (horde spawns will be relentless and last longer)
-closer to vanilla spawn types (hive tyrant, alpha lictor, random space marines and allies should no longer spawn. there will still be guardsmen and dreadnought spawns. might need some testing)
-bot loadouts (bots have been given closer to tabletop loadouts - Tacticals have regular bolters and knives, Assaults have heavy bolt pistols and chainswords, Heavies use heavy bolt rifles and knives, Reivers use heavy bolt pistols and knives, Snipers use bolt sniper rifles, etc)
-larger bot squads (10-man tactical/assault/heavy intercessors squad, 5-man bladeguards, 5-man snipers/scouts etc)
-bots will now always use the mastercrafted skin for their weapons regardless of difficulty (it's odd for a while squad to be armed with relic weapons)
IMPORTANT: When playing as one of these squads (except the one-of-each squad), you will be forced to play as only those specific classes (eg. you can only play as a Tactical if using the 10-man tactical squad settings). If you try to switch to another class in the armory, you'll be put back to being a tactical. This is just how the game works.

INSTALLATION:
MAKE BACKUPS BEFORE INSTALLING
ASTARTES OVERHAUL IS RECOMMENDED FOR FULL EXPERIENCE
With how modular this mod is meant to be, having pre-packed files for each possible variation is just unfeasible. Manual installation through file merging is the ideal method for getting this to work. Pakcacher is a necessity. WinRar is ideal for opening the .pak files, but I understand that 7zip also works.

For the Spawn Rates:
-install Astartes (ignore this if not using Astartes)
-download SpawnSystem
-go to "Space Marine 2\server_pc\root\paks\server\default" and open up default_other.pak with WinRar
-drag the ssl folder from SpawnSytem into any open space within
-close the pak files, and drag default_other.pak onto packcacher.exe to rebuild the cache file

For the Squad Types:
-install Astartes (ignore this if not using Astartes)
-download Squads
-go to "Space Marine 2\server_pc\root\paks\server\default" and open up default_other.pak with WinRar
-choose which type of squad you wish to play with and open its rar file
-drag the ssl folder into any open space within
-close the pak files, and drag default_other.pak onto packcacher.exe to rebuild the cache file

UNINSTALLATION:
Delete default_other.pak, their respective cache file, and bring back the original files that you saved from before installing. Otherwise, you can also verify integrity on Steam.

IF YOU ARE A GLUTTON FOR PAIN:
Visit us in the Increased Spawns and Larger Squad Sizes showcase thread over at the Astartes discord where other users have posted their own settings. Some of these can range from simply more common Extremis enemies while others have made it so that you're facing against 3 or more carnifexes at once... Check the pinned comments for the files.