If you want to increase production at your factories, you can always add more slots and shelves.
You can have a maximum of 7 slots per shelf. To buy more slots with Township Cash, tap the + icon to the right of the slots you already have.
You can have a maximum of 8 shelves. You can buy shelves for ingots when upgrading your factories at the Academy of Industry (starting from Level 22). To add a shelf, go to the Academy of Industry or tap the star icon in the upper left corner of your factory window, then tap the shelf icon and tap Upgrade.