How will Moshiach teach Torah to the entire nation at once?
In the Messianic era, the time and effort required to understand Torah will be drastically reduced.
The style of learning in the Messianic era will be different from anything that has been known until now. In the future, spiritual concepts will not need to be explained in words in order for us to grasp them. Our physical eyes will be so refined that we will be able to actually sense and visualize these concepts. It will take only moments for us to comprehend the deepest ideas in Torah. Today, only a select few are truly capable of grasping these concepts, and only after many hours of effort. In the Messianic era, the time and effort required to understand Torah will be drastically reduced. Thus, there will be no problem for millions of people to listen to Moshiach's Torah, and each will absorb it according to his or her level.
Just as an example: Imagine that you are describing a work of art to a friend. Think of how long it would take to describe every detail and nuance of the painting. However, if you simply show your friend the painting, he will absorb all the details at once.
Similarly, when Moshiach comes we will actually perceive G-dliness. We will instantaneously absorb spiritual concepts, and won't need to have them explained to us.
Although this type of spiritual perception seems far from our reality, a select few scholars had this kind of vision. For example, the famous Kabbalist Rabbi Yitzchak Luria once told his students that during his half-hour Shabbat nap, deep secrets of Torah were revealed to him. He said that it would take him sixty to eighty years to expound on all these secrets to his students.