the rune must be visible and focused on when activated, so only one may be activated at a time, unless linked, even if they have the same key. I demand 10% activity per rune and include them in the three actions per melee limit.
you may want to refer to Treasure Companion for the nature of keying items.
Items can only hold a certain number of runes based on surface area. The last time the list was in the party, I required items enruned needed special preperation/construction. Rune Blades, Rune Armor, Rune Shields, Rune Stones, Rune Wands, Etc. Magical items innate aura prevented enrunning abd disrupt the energy weave of rune magic, or so went my logic. It worked well enough.
Still, the runes are one shot and done. Let the player have fun with the list. You as GM can always through in a few small battles were players will use up their runes. Lets face it, players aint that smart when it comes to trying out a new toy. Then there are those new monsters, drawn to feed on the arcane and only hurt by rune magic. After such an encounter, players will show restraint, fearing one more arcane elemental might appear.
An enruning charm canbe introduced into your game. These magic items have a limited arcane inteligence that cast a specific spell, or a group of spells, to recharge runes. Essentally sophisticated daily items with charges that can ONLY be used to recharge runes.
What ever is FUN, thats what i recommend.