Less than I should, more than I want to, used to be the answer here. While feedback is great, playtests are a profoundly uncomfortable experience. It’s literally people wandering around inside your head, turning stuff over, occasionally setting fire to things and wondering why you went with that shade for the walls.
But, in the ongoing After The War design process, I’ve made my piece with it. The playtests we’ve had have been uniformly brilliant and massively useful. So now, the answer is ‘as many as we can get and maybe buy them donuts too’.