There are four summons to acquire, but you won't get the last one until mid-late game.
1. Moses: Acquire by completing the first Church mission and battle
2. Gerald: After you get Fartkour, go to Kennys hour and Fartkour to the top, you'll see Gerald there. Speak with him to unlock the Cheese Vial summon.
3. Jimbo & Ned: Speak with them, then find the wallet on one of the tables at the Peppermint Hippo. Return with it to get their summon
4. Classi: The last one you will get. Story-related after you beat The Thin White Line quest
Once you acquire the last one, you get the trophy