Saying it's a spectrum implies to most people there's some sort of Gaussian distribution and there's not.

There's not like 20% of humans with mammary glands and a scrotum, right? Or 10% with no reproductive organs. Or 15% with both sets.

The obvious flip side of 1 in 1500 is that 1499 out of 1500 are binary.

So there's not really a spectrum as most people would understand that word.

The word you're looking for is "bimodal distribution".

The spectrum of sex characteristics is a bimodal distribution with two peaks. and the vast majority of all humans fall close to one of the two peaks peaks. however, they indeed exist medically classified circumstances for bodily sexual expression that is not on one of the peaks but somewhere in between.

and the percentage of those not close to the peaks is heavily contested and varies between 1 in 15000 ( putting extremely high bars on "uh wen can't tell") to 17 in 1000 (counting for example a larger clit as a penis-y ambiguous thing)

bimodal distribution with distinct peaks and a low but non-zero density between the peaks.

To add to the other reply, nearly 100% of people classified as men at birth do have mammary glands and can lactate if they take certain medication. Some men can lactate without medication.