Necromantic Scroll (dropped by Mourning Wood during the Pumpkin Moon event)

Papyrus Scarab (crafting from Necromantic Scroll and Hercules Beetle)

Pygmy necklace (can be purchased from a Witch doctor for 20 gold coins at night)

With full set of stardust armor you will have 5 extra summons + 1 stardust guardian

Summoning poison will give you +1 number of summon



Bewitching table also add 1 extra summon (you can find it in skeletron dungeon)


in the end you will have 11 summons + 1 stardust guardian.
Btw it's useless because you can do it only after killing moon lord.