A Strongbox is crafted at the lowest tier (Basic). It can be upgraded to higher tiers using Upgrade Kits. As it is upgraded it is able to hold more items.
Number of Slots
This amount can be increased further using the Holding enchantment, which increases the storage capacity additively. Each level of the enchantment increases 9 more slots to the maximum capacity. The maximum storage capacity possible is 126 slots for a Resonant Strongbox with Holding IV.