How much RAM does the Dell OptiPlex 5400 All-In-One support?

The OptiPlex 5400 All-In-One supports up to 64GB of memory across 2 slot(s), up to 32GB per slot.

What type of memory does the OptiPlex 5400 All-In-One use?

It uses DDR4 SODIMM memory modules.

Compatible drive types: NVMe M.2 2230 (PCIe Gen 4) SSD, NVMe M.2 2280 (PCIe Gen 4) SSD, SATA 2.5" SSD, SATA M.2 2280 SSD.

Dell OptiPlex 5400 All-In-One Memory & Storage Upgrades | Seifelden

Good to know

Maximum capacity per slot is inferred from total capacity ÷ number of slots.
Compatibility is based on published specifications, not yet confirmed against a manufacturer tool.