25,000 words in total (and about 3,000 different words) is what the seven volunteers who have participated in a study conducted by scientists at the University of California at Berkley heard. While listening to the words, the volunteers they remained connected to a functional magnetic resonance machine.
The purpose was to study which parts of the brain were activated based on words and thus build a 3D model that has been called as PrAGMATiC (probabilistic and generative model of areas tiling the cortex).
As you can see in the video that heads this post, this model is able to predict how the brain will react to certain words. The funny thing is that, depending on the word, an area of the brain was activated more. And that these activations were thematic: For example, the green areas are those related to the visual, the light blue with the touch, the roses with violence, the oranges with people, etc.
The sample size is small, and adhered to the English language, but it is a first step to devise a system that be able to read the thought.