That was my experience, too. I knew of the existence of "transsexuals", but I thought they were rare, like maybe a couple of dozen in the world. That's how I managed to talk myself out of knowing that I was trans: there was no way I could be that weird, could I?
Encountering my first real transgender person and realizing that she was just a normal person carrying on a normal career was a real eye-opener. That, and the Caitlyn Jenner effect, opened my eyes enough for me to start investigating.
And yes, I was surprised at how many of us there are. I would never in a million years have guessed that I could sit in a crowded room full of trans folks, all just from one small city.
I am also surprised at how many of us are only figuring it all out in late middle age or senior years. I would never have guessed that either.