A Cache 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.
This amount can be increased further using the Holding enchantment, which increases the storage capacity multiplicatively. Holding I increases the storage x1.5, Holding II x2, Holding III x2.5 and Holding IV x3. Therefore the maximum storage capacity possible is 1,500,000 for a Resonant Cache with Holding IV.