The PlayerInventorySaveData.json file stored all the items you collect in the game, you can edit the count to incrase or decrase the items.
I tried to find the most item's ID, when you input the ID, it will display the item name.
Here is the item ID list, if you want to add a new item, you can find the ID here. Some item name may not correct due to the translation issue, English is not my mother language.
The save files can be found in this path, you can copy the path and replace the username and Steam ID to yours, then paste it in the file manager to open.
If your computer has only logged your own Steam account, that should only have one child folder in your "Saves" folder. If there are many folders, you can find your Steam ID on SteamID.IO website.
C:\Users\<Your Username>\AppData\LocalLow\Endnight\SonsOfTheForest\Saves\<Your Steam ID>When you open this folder, there will have up to 3 child folders inside it, the "Multiplayer" stored the files when you are the host in multiplayer game, the "SinglePlayer" is for single player game.
Choose the game type you want to edit and open it, then you may see there are many child folders, you can sort them by modify time to find the latest save.
Please select PlayerInventorySaveData.json file