In Disgaea 5, there are a total of 12 unique Evility slots. Each character has access to two of these slots, for a total of 24 slots.
The game does not allow you to purchase or create more than 24 unique Evility slots.
To get more unique Evility slots, you must first complete the main story. After that, you can access the New Game+ content, which includes an additional 12 unique Evility slots.
These must be unlocked by completing specific requirements, such as defeating a certain number of enemies or clearing certain maps.
Once you have unlocked all 12 of the New Game+ Evility slots, you will have a total of 36 unique Evility slots. This is the maximum number of slots that any character can have.