In many ways, the problem is unanswerable because it's a question of degree. Some people may crossdress for a short while in their life, or very occasionally, or merely wonder about expressing their gender differently; others have to transition fully.
|and a few who transition|
I've heard figures of 1 to 1.5% bandied about. It's impossible to know, of course, since what is clear is that the overwhelming majority do not go out of the house or live a day-to-day life in their chosen gender. I'm unusual in that I do, but given how hard it is to meet up with other like individuals, I can only conclude that those at the publicly-visible end of the spectrum are few and far between. I call this phenomenon the Transberg - like an iceberg whose bulk is almost completely hidden underwater, where passing shipping can't see it. (I'll draw a picture of a Transberg one day.)
Britain has a population of 63 million (officially). Statistics seem to show that a few hundred a year transition here, and that the total number who live after transition seems to be 6000 - 8000 individuals. Or roughly 1 in 10,000 of the population. But given the earlier statistic I quoted, this is only about 1% of the total estimated (i.e. guessed) trans population.
Various people have had a stab at assessing our numbers. Some of those people are driven by an activist/political agenda which makes their statistics suspect, rather like reading tobacco industry funded research into smokers' health. Others use intelligent guesswork based on what appears online. My own feeling is that gender is fluid and changes throughout an individual's life so they may fall in and out of the definition at different times (or would they still count as trans in perpetuity?). As I've mentioned here before, I've been trans for ever, but the degree of it does vary from year to year and season to season. One of my own sisters was a real tomboy when aged about 4, and she can be a little boyish in style even now. But is that a sign of being trans? She's always had some feminist views in her teen/adult years and her current style may be a demonstration of those, rather than any trans aspect manifesting itself.
It's irritating not to know the answer to this question as it would help inform society. But given the fear of ridicule most trans people in the West have, which keeps them hidden away at home or in hotel rooms, rarely even telling their spouses, family or friends, I wonder if we will ever be able to know.
Why is it important to know? I think it helps demonstrate that we're not rare or weird. Supposing we could say, "Look, this school has 500 pupils aged 13-18, so by the law of averages 5 of them will be trans, one in each year". Or, "This football stadium is filled with 70,000 fans. Seven of them are transitioning/have transitioned and seven hundred are trans in one way or another". Just by way of examples. It makes the reality more immediate to everyone. And shows that the likelihood of a trans person living in your street, working in your office, attending your college/club/church etc. is quite high (or possibly quite low, depending on the elusive answer).
Here's a science article (from the respected journal Nature) mainly about intersex conditions. It offers tantalising glimpses into potential explanations for intersex and even trans conditions (none of them new). But as some of the readers' comments rightly point out, a few cases (almost anecdotal) do not a theory make, especially when intersex activists wish for an outcome and use the slenderest evidence to back their ideas.
There really is no comprehensive scientific literature on trans matters at all, involving large-scale, double-blind trials and robust conclusions, just these little snippets of possibility. We are therefore mainly left with some rather old conclusions of psychologists, who influence our policy makers and health professionals, and who think transness is a mental disorder (and some of my trans friends agree with this, including a biologist whose opinions I necessarily respect). Like most trans people, though, I don't think my trans status derives from my thinking or mental state at all - though there's obviously a two-way communication going on between my body and my mind - but from something fundamental and innate. But how can I know? How can I be sure? As yet, I can't, and this is frustrating as I'd like to say to people "Look, I'm trans because ... my hormone balance/genetic makeup/mental state/etc. ... is such-and-such." I did take part in a study some years ago that yielded some interesting results (basically, we trans folk each develop through life in a very similar way, despite being distanced from one another, which suggests that transness is something innate rather than influenced from outside). Sadly, the persons directing that study also have strong political views and a trans promotional agenda so - like those tobacco industry researches - their conclusions, even though properly peer-reviewed, need to be considered cautiously.
I remain frustrated in my quest to know why I am trans. Frankly, this condition/existence/state/manifestation - describe it how you will - has wrecked my life, not because it itself is a problem, but because people in my society (including me) - and, I guess, in just about every other society - don't know why this trans 'thing' happens.
All I can say at the moment, like Stonewall, is "some people are trans - get over it!" But that's also a failure: a failure to understand, to research, to conclude, and so to support in any real way.
I'm trans. I live with that. I wish I wasn't. I live with that, too.