The maximum number of artifacts that anyone has ever obtained without editing some in is 18. You may be lucky and find some in chests, but the main way you get them is by looting high-level monsters: the big dragons, the great wyrm, and the supreme titan, mainly. When these stop arriving, you go and collect the 5 placed artifacts: Excalibur (rock in Eel-Infested Waters), Mordred (quest in Free Haven) and Morgan, Igraine & Guinevere (obelisk quest). But sometimes the game stops giving you items before you reach the 13 random ones, and nobody knows why - or if he does, he not saying.And about the chest in Blackshire, you're luckier than one guy some years ago. He reported that he did the quest, went for the reward, and the chest was empty!