You can find at most 13 random Artifacts and Relics before the game stops providing them. Any one item can be found once and only once, so you can't get 4 Poseidons and 4 Hadeses without using an editor. The catch is that if you find and take any of the 5 placed Artifacts before you've taken 13 random ones, it counts against your allocation, so in practice, you can end up with any number of Artifacts and Relics from 13 up to 18.

The 5 paced Artifacts (the sword Excalibur, the dagger Mordred, the amulet Morgan, and the two rings Guinevere and Igraine) are found in specific places, and are provided independently of the random Artifacts and Relics, so these are the only items that you can legitimately have two of.

For ordinary weapons, I'm fairly sure that the sword/spear combination is optimum for melée combat in turn-based mode, but the Artifacts and Relics are different, so those need to be taken on their own merits. However, in this game, and I'm pretty sure in MM7 as well, the best weapon in real-time melée combat is unquestionably the blaster. It doesn't do much damage in any one shot, but it fires so quickly that the damage done over a given time-period outweighs any other weapon. But they're useless in turn-based mode.

Having said that, I still consider that the most effective combat technique in MM6 is the spell Shrapmetal in contact range, cast by a fully developed Dark Magic master equipped with Guinevere and a Ring of Dark Magic. Each of those multiplies the effectiveness of the spell by 1.5 (with the obtained damage figure rounded down if it's not a whole number).