Originally Posted by Drip

Though, keep in mind that if you want to equip all your girls, you'll need at least as many jewelry slots as you have girls.

Why would you need equipment for all girls? You can only ever field 20 of them, so having 20 of each of the four item types is enough. When you swap a girl, you swap her equipment together with her. That leaves you with 20 slots for building your upgrades with. Maybe a handful less for the slots taken by the unique items existing.