This debate butts up against the limitations of language and it is necessary to take care to define terms. This definition will be more or less useful depending on what you are trying to communicate.
“Sex” could be interpreted to mean a statement about chromosomes, in which case there are more than two variants.
Or it could be interpreted to be a statement about reproductive potential, in which case the “two sexes” model is a better fit, because there’s no intermediate gamete between sperm and egg. Do intersex conditions break this model? Almost always not: the variations of sex development are variations on the male pattern and the female pattern. Are there exceptionally rare individuals that defy categorisation? Surely there are, but the existence of extreme, rare outliers doesn’t invalidate the model.
Which definition is most useful? In the context of women’s rights, it’s clearly the latter. Women suffer discrimination based on their reproductive biology, not based on their chromosomes. It’s also the model that the vast majority of people use as their everyday working definition of sex.
So I think anyone who wants to claim there are more than two sexes must accept the responsibility to qualify their terminology.