Exact estimates vary, but are generally in the ballpark of about 1% of people being trans and non binary (though this is skewed towards younger people).
At any given time, the US federal government has 535 elected seats. The point of this comment is to highlight the disproportionately low rate of trans people getting into politics and being elected. Things are stacked against trans people, whether it's the barrier from how poor social acceptance growing up can limit opportunities or heightened fear of getting into politics because of how toxic they are.
And we're talking about a job where their coworkers are sometimes actively campaigning to outlaw trans treatment, acceptance, and protections. Something that in many work places would get you instantly fired, but is permitted in US politics.