This will roughly double enemy spawn counts in each pod. It doesn't double the leader, so if a pod has one officer and two advent, it will have one officer and four advent. This makes things a lot harder so it's expected that you've setup some other mods to give you an edge and make things possible.
Fair warning I have not extensively tested this. It's possible increasing enemy counts messes up some scripted missions or something in the parts of the campaign I haven't yet seen.
If you get a crash you think is from this mod, it would help if you can post a comment with a link to your save file.
Manual Install the .INI file if you wish:
Backup your XComMissions.ini
Download the mod file and unzip.
Copy the .ini file into \Documents\My Games\XCOM2\XComGame\Config
Put your original missions file back to remove.
Or copy the contents of the mod download to the directory \Steam\steamapps\common\XCOM 2\XComGame\Mods\