I'm currently making a game, does anybody know how to reduce the max inventory space?

My solution would be to check when a child is added to the player's backpack (https://developer.roblox.com/en-us/api-reference/event/Instance/ChildAdded) and if there is already something in the backpack, put the tool back into the workspace or whatever you need to do with it. That's how I would do it but I'm sure there are many other ways if this way doesn't work for your game.

Just know there's no setting for it, you have to manually script a system for this.