Get Norbyte's extractor tool (EE Pak Extractor thread), and unpack Main.PAK, or Game.Pak, or one of those. Don't remember which one exactly has the right folders. But when you find one with Mods/Main/Story/Rawfiles, you got the right one. Pretty sure you'll need to edit story.div.osb and/or story.div.osi. Not sure why there's two files, but I guess just edit both? You're on your own from there, because I don't know if hex editing those files will work, or if the changes will take. You might consider finding another really obvious thing to change in case the max companion count is also determined by something else, and you can't tell if your change just didn't work or if it's been overridden.
