Spirits come from the Earthen, Spiritual, and Astral Realms. Many spirits are a transitional form of mortal entities who have lived and died in any of the Realms, but there are also Immortal spirits.
Our world has existed for billions of years. During that time, billions of entities lived and died on our planet. When they die, they transition into spirit form. The worlds that exist beyond the Earth and our solar system have also existed for billions of years. These worlds contain their own realms, dimensions, and beings who have lived and died as well. Those beings have also transitioned into spirit form. Additionally, some of these realms accommodate Immortals who have no expiry on life.
If you then consider the trillions of various forms of life that have existed in every solar system within every galaxy, you being to understand the unfathomable number of life that has lived and died within the Universe. There are countless species who have lived and died, creating decillions of spirits across the endless expanse of time and space within the 3 Realms.
They come to us as unbound spirits via several means; through conjurations for binding, summoning for temporary contact, invocations for long-term contact, visions, extra sensory means, and more. Between the spirits and entities from our own realm and those existing other realms, dimensions, and galaxies in the Earthen, Spiritual, and Astral Realms, there are countless spirits accessible for contact and interaction.