Adult socialsex charting
So nature doesn't decide where the category of "male" ends and the category of "intersex" begins, or where the category of "intersex" ends and the category of "female" begins. Humans (today, typically doctors) decide how small a penis has to be, or how unusual a combination of parts has to be, before it counts as intersex.
Humans decide whether a person with XXY chromosomes or XY chromosomes and androgen insensitivity will count as intersex.
By the way, because some forms of intersex signal underlying metabolic concerns, a person who thinks she or he might be intersex should seek a diagnosis and find out if she or he needs professional healthcare.
If you're curious about how common intersex conditions are, go to the FAQ called "How common is intersex?
Breasts, penises, clitorises, scrotums, labia, gonads--all of these vary in size and shape and morphology.
":/faq/frequency To answer this question in an uncontroversial way, you'd have to first get everyone to agree on "what counts as intersex":/faq/what_is_intersex --and also to agree on what should count as strictly male or strictly female. How small does a penis have to be before it counts as intersex?