Stuttering isn't very common in adults. Only about 1% of the world stutters. Not only that, but a lot of stutters are so mild as to be almost completely unnoticable. I've met a few people that say they stutter - but I didn't notice it before or after them telling me that. Perhaps their stutter is only evident in extremis, perhaps they're very good at managing it. Either way the number of people with noticable stutters is likely quite a bit less than 1%.