Not often. My daughter rides and she's been thrown a couple of times. Every time it was a case of the horse refusing a jump and stopping short. Fortunately every time it was in a ring with a soft dirt surface.
That said, I cannot count the number of times I've gone one way and my horse has gone another. (The stage-combat training I've had has taught me to fall without hurting myself, and that's helped.)
The vast majority of those are due to me signaling to the horse that I'm gonna do something, and then not doing it or doing it wrong. Only a couple times was it the horse's idea to ditch me.