collect up to 13 A/Rs from random chest generated placements and looting high level monsters. If you collect 13 randomly generated A/Rs before collecting the 5 placed A/Rs you can have a max of 18 A/Rs. If you want you can get one duplicate of each of the placed A/Rs. A placed A/R collected before you already have 13 will count as 1 of the 13. Selling an A/R doesn't allow more A/Rs to be generated and that A/R is gone for good. Chest contents are generated when you enter the zone or dungeon for the first time.There are seven zones (MD, KS, BS, DS, HI, PV & SW) that have chests that may generate A/Rs, some more than one chest. Castle Darkmoor Aerie chest as well. It is possible to open a chest with an A/R that doesn't show up but still counts as 1 of the 13. I recommend saving before opening a chest, open chest, see contents (maybe make a new save), then reload again several times to see if there are other items. Grayfaces patch allows opening a chest, emptying it, then opening it again to see if there's something else to collect.
If there are specific A/Rs you want to have, I recommend selectively shopping at a friendly high level corpse. I like to collect the doubles of 3 of the placed A/Rs and about 6 other A/Rs.