I've read that recent research suggests that we are about 1 in 3,000 people, and occur at a uniform rate regardless of the country/culture, and that numbers of transgendered Native Americans recorded by white 'explorers' (exploiters) a few hundred years ago revealed them to be a similar proportion of that population.
One in three thousand far less than 1%, but it is quite a lot, really -- something like fourteen trans people would be there if you filled every seat in pro-sports stadium with randomly selected humans. In an average high-school there are few enough students that you will probably eventually recognize every one of them on sight, and know the names of most, and statistically, one or two will be trans.
I think we know about it more because trans people now (like native american transpeople in the past) know that they are trans since it is now recognized (if not accepted) in our society.