Your Warehouse usually starts with 8 slots. Each unique item takes up a single slot, but duplicate items stack nicely, taking up no extra space.

If you have 99 Airplanes, they only take up one slot.

A single Gold Doubloon takes up as much space as those 99 Airplanes!

No, it doesn't make much sense, but that's how these warehouses work.

Under special circumstances, your warehouse might Overflow.