How to decide whether or not to make story a series or just one book?

My story is a bit complex due to the character's and their many layers as dead people in the afterlife. They each have their flaws and desires but they are often thwarted by obstacles, then of course they all have the plot as their common problem that bonds them together. I don't know if I want to make this all in one book or make it into at least two or three other books like a series. How does one go by making this decision?