I've seen hundreds. Being away from the ground means so much less light pollution, so many that would be invisible from the ground are visible from the air. This is especially true when flying over less populated areas. I saw a geminid a couple of years ago while flying over Eastern Oregon which seemed to track half the sky. The Geminids are probably one of the most impressive showers if you have clear skies in December.
Of course, the UK is horrific for light pollution, there are many places in the US which have a good view from the ground.