Upgrade Mac Pro 5.2 2012

"to its max" is extremely vague, especially as you didn't really tell us much about your existing configuration.

But here are some 'max' specs you can run on a 2012 Pro

• Max RAM - 128GB in dual channel or 96GB in triple channel. ~$400 (but only if you have a dual CPU) - [OWC]
• Max Graphics - Nvidia GTX Titan X or AMD RX 5700 - [macvidcards] $675 - $800
• Max OS - generally High Sierra with NVidia or Mojave with AMD.
• Max Drives - 7. Four in the regular bays, one above the optical & two on a PCI card - [Mac Pro PCIe SSD Hardware Options]
• Max CPUs - dual 6-core Xeon 3.46GHz - [OWC] $1,379
• Max Bluetooth/WiFi - [macvidcards] $175
• Max USB - 4-port PCI USB 3 card - [innatek] $30 (but many of these type of card are incompatible with newer OSes 10.10+))