If a card specifically states it affects all Duelists, Monsters, Spells, or Traps, then it affects both Teams (all 4 Duelists.) If a card affects "both Duelists" or "each player," then the Duelist activating the effect and the opponent sitting directly across from them are affected
Argentum Armor [Cat deck]: Late game in commander I can see this being really good
1116 del Cdigo Civil y Comercial
The number of duplicates that must be used will depend on the rarity of the card the rarer the card, the more dupes you will need
After all, Terastal Pokmon, which now flood the meta, are blessed with the unique ability that prevents them from being damaged while on the bench
This commander can reveal X-spells to add a bunch of mana (including revealing the card you want to cast, which seems almost like cheating!)