AF armor requires certain quests. every job can recieve their first AF quest (a weapon) around level 40 called AF1.
After completed AF1 and reach level around 50, you can recieve the next quests. AF2. Completed AF2 and you can recieve AF3, AF4 and so on for each piece of armors. But beware, AF quests are not easy. A PT of 6 level 57s got wiped trying to kill the boss for AF3 WHM before
AF armors can't be traded or re-do. You got it, you got it. You can throw it away ofcourse if you choose to hehe. All pieces will require level 50+ to equipt (some pieces up to level 59) except the weapon AF which requires only 40-43.
if you have the patience to bring more then 1 job up to 50's then yes you can have more then 1 AF set. ...... again, if you are patience enough.... And you'll need to level to 59 in order to equipt every AF piece you got