It is possible to overflow the item card limit on a deck. What happens in this eventuality? Let me give you an example. My life currently ha:
16 item card slots (Tatyana's Lost Tarot Deck) 2 Chromatic Blast item cards (Shard Warrior's Helm) 2 Shadow Trap item cards (Undying Malistaire's Tunic) 0 item cards from boots 6 Ultra Torment item cards (Eye of the Soothsayer wand - not the max spells for a wand either. I've seen at least 7) 2 Lifeblade item cards (blade of the felled titan jewel slot) 3 Life blade/Sprite swarm/Hammadryad item cards (total, not apiece, on my Mega Dryad pet)
0 item cards from my deck
So, let's do the math: 16-2-2-0-6-2-3-0=1
If I change my deck AND equip it with a triangle jewel, and use boots from waterworks, that's 3 more item cards. What happens to the 2 remaining item cards that can't be displayed?
They will be available to cast and the display of the item cards will include an arrow that points the right, which, when clicked, will scroll the display of item cards to allow you to disable them if you like.