Stack Size is the number of identical items that can fit into the same grid slot. For most resources, this number is 20. For bigger resources like Gears and Steel Sheets, the stack size is 10. MREs stack to 5, and First Aid Kits, Repair Putty, and all repair kits stack to 3. Items such as large craftables (panels, doors, batteries, etc), decoration items (decals, spray paint, garage deco), tools (Scrappers, Impact Hammers, Prybars, etc), and Hands Only items will not stack.