I don't know if there actually is a way to determine how many people are left-handed. I don't even know if there are places that are still punishing and trying to change lefties today. I know my grandfather was left-handed, punished and forced to use his right hand. This left him with a stutter and caused him to be left-back in kindergarten. He did outgrow the stutter. When my mother turned out to be left-handed, he went up to the school and insisted that they leave her alone and let her be left-handed. She really loved learning how to write on those half-desks they used to use. She wrote with her arm hanging off the desk. Neither or them used that weird over-arm way of writing, they both had very neat, though small, handwriting. My daughter is left-handed and no one even suggested that it was an issue. None of them are Wikitreers or have any interest in genealogy. My daughter is the only grandchild of my mothers who is left-handed.
I am ambidextrous. I can use either hand, but I don't know how much is nature vs. nurture, since I had the left-handed mother. I usually write with my right hand, but I can write pretty well with my left. I can also write backwards, upside-down, any direction really. Funny story. When I was in the Army, they wanted to teach us to write on one of those clear boards. We would have to write back-wards so that those on the other side could read it. I was like, "Teach? Doesn't everyone know how to do that already?"