I have sucessfully increased the number of adventurers you can create and it works perfectly with current saved games without any issues.
First, download dnSpy - https://github.com/0xd4d/dnSpy
Scroll down to the "Binaries" list and download the latest release.
Extract the .zip to any folder and run dnspy.exe (64-bit) or dnspy-x86 (32-bit), on your OS.
My OS is 64-bit - so I use dnspy.exe
Next make backup copies of both Assembly-CSharp.dll and UnityEngine.dll - from wherever you installed PoE open "PillarsOfEternity_Data/Managed" or whichever folders were used during install.
Now, in dnSpy click "file" "open", find Assembly-CSharp.dll and also "open" UnityEngine.dll - wherever you installed PoE...
Open Assembly-CSharp.dll and scroll down and open "PartyMemberAI", then from the drop-down list find the "MaxAdventurers" entry.
Right-click on "public static readonly int MaxAdventurers = 8" and select "Edit Class C#" and simply edit the number to your liking. I choose 19 to make a grand total of 30 characters.
Now, click "compile", the pop-up will auto-close when it is complete. Then exit dnSpy.
You will be asked to save the file and give it a name - do so. Done!
Replace your default Assembly-CSharp.dll with the new Assembly-CSharp.dll (you must rename it exactly as Assembly-CSharp.dll from whatever name you had saved your edited version as in dnSpy) - now start PoE.
Remember that tons of modding will always slow a game down so choose wisely based on your hardware capabilities.
I have had no issues with gameplay, sound, graphics, saves or slowdowns. Enjoy.
To my knowledge it does not break achievements -
Edited by blackstone777, 23 September 2018 - 01:50 PM.