If you define 'transgender' as those who are actively working towards reassignment, or who regularly present as the 'opposite' gender then the proportion of the population is probably fairly low.

I wonder what it would be in a less hostile world? How many people currently unwilling to admit because of shame or fear - even to themselves - that they're unhappy with their gender might take steps to put things right?

Not that numbers should matter. Those who constantly try to belittle the problems transgender people face by claiming they affect so few only emphasise their bigotry. The smaller the minority, the greater the responsibility society has to ensure that they enjoy the same quality of life as everyone else.