So I have a habit of wanting to hoard things and characters are no exception. My first problem is having enough jail space to hold the newcomers and those I want to keep for taming. Which folder and which line of code should I look in to increase to the potential number of rooms available or is there a mod already for that?
There isn't any mods that increase the limits on rooms because the Patreon supporters panel includes a cheat that effectively removes the limits, the code for mansion upgrades is a little bit convoluted compared to other parts of the game, and before recent versions the game would lag and crash with relatively low numbers of slaves.
The current version of the game is less prone to problems, especially if the progress is never loaded from a save file, but I suspect that the game would still crash when loaded from a save with somewhere between 50-100 slaves. It's been a long time since anyone mentioned running into that problem, but the fixes were aimed at reducing lag not preventing crashes so it should still crash.
If you still want to tinker with the limits, then the text file ".../files/scripts/mansionupgrades.gd" in the program folder will contain the level limits for upgrades. I recommend getting the Debug mod if you are editing the files as it makes it easier to deal with typos. Also note that applying mods in-game will erase any edits made directly to scripts.
