Your maximum possible AF is (1 + absorb)*(your level)*10. You can achieve this via an AF buff or by wearing a complete set of level 50 epic armor. For Midgard this AF cap for chain wearers is 635.

Level 51 armors (attainable once you reach rr5) that are craftable allow you to reach an effective af of 647. For comparison, plate wearers can reach a maximum of 683