Unlike Eidolon Passive Skills, Weapon Skills for Sub Weapons (サブウエポン) are also in effect, and a full list of all Weapon Skills applicable to any character are visible when clicking the Skill (スキル) button in Formation.
As all equipped Weapons contribute their Passive Skills, the maximum number of Weapons worth keeping for Passive Skill usage is 60 - 10 per Element. Since the default weapon limit is 50, it may be worth considering increasing this limit to at least 70 in the long term due to this effect, but at present no content is difficult enough to even require a fully upgraded 3★R arsenal, let alone weapon skill stacking