Since 2015’s Summer update, Spells are assigned a housing space number indicating how much storage space they take up. 

The number of Spells you can hold at a time depends on your level of Spell Factory, and whether or not you have a Dark Spell Factory.

At level 1, your Spell Factory contains 2 housing spaces for Spells. That number increases by 2 every time you level up the Spell Factory, and it maxes out at level 5 with 10 housing spaces. The Dark Spell Factory adds 1 housing space, regardless of what level it's at. This means the maximum spell housing space you can have is 11 in total.

Regular spells take up 2 housing slots each, whereas Dark Spells only use 1. That means 11 housing spaces can fit a maximum of 5 regular spells and one Dark Spell, or 3 regular spells and 5 Dark Spells.

Mix and match for maximum dev