Only two sexes occur in most species, but Physarum polycephalum, a common yellow slime mold, has over 500 different sexes!
The Facts of Life
A true mature slime mold is one gigantic cell--easily visible to the naked eye. Unlike what we typically think of as a cell, however, a smile mold 'cell' consists of cytoplasm with millions of nuclei. When it comes time to reproduce, a stalked fruiting body is produced which releases spores into the air. These spores develop into sex cells.
Slime molds make sex cells that are only one size (isogamous); when size is the same, other features determine cell sex. In slime molds, it's three genes, matA, matB, and matC (all of which contain variants) that are responsible for determining sex.
A mature individual has two copies of every gene and is capable of producing 8 types of sex cells. When one counts all the different combinations of variants of matA1-13, matB,1-13 and matC1-3 available, you get more than 500 potential different sexes!
For successful reproduction, a spore must simply find a partner that has different variants of the three genes. What makes slime molds truly successful, however, is that the cytoplasm of the offspring is merged from both parents. How's this work? Mitochondria are only inherited from one parent; there's a hierarchy within the variants of matA.
Judson, Olivia. (2002). Dr. Tatiana’s Sex Advice to all Creation. New York: Holt Paperbacks.