Orb slots are a feature of The Defect, which limits how many Orbs you can have channeled at once. If you channel more Orbs than you have Orb slots, then the rightmost Orb will be Evoked to make room.
The Defect begins with three Orb slots and considers the rightmost Orb to be the active Orb, being the one that will get Evoked first. Other classes begins with 0 Orb slots. They will gain 1 Orb slot once they are able to obtain Defect cards from card reward, usually through 
Prismatic Shard or some game modifier.
Player can increase and decrease the number of Orb slots by playing cards, possessing some relics, or drinking 
Potion of Capacity. Losing a slot with an Orb equipped does not Evoke it. A character can have at most 10 Slots. If a character has 0 Slots, Orbs can no longer be Channeled. Orb slots do not carry over between combats.
Related[]
Cards[]
These cards modify the number of the player's Orb slots. Numbers in parentheses are for upgraded versions:
Relics[]
These relics modify the number of the player's Orb slots, or interact with an empty Orb slot.
Inserter: Gain 1 Orb slot every two turns.- Runic Capacitor: Start each combat with 3 additional Orb slots.
- Frozen Core: Channel 1 Frost per turn if there is an empty Orb slot.
- Prismatic Shard: Start each combat with 1 Orb slot if you are not playing Defect if Base Mod is installed.
Other[]
- Potion of Capacity: Gain 2 ( 4) Orb slots.
