The current number of user camera feeds that you can see at one time is currently limited to 9 in Microsoft Teams. (Microsoft is working on expanding this number in the very near future)
However- it is highly recommended to not request students to be on video when they are watching a lecture or presentation. Displaying video feeds takes bandwidth away from the session and the amount of data increases exponentially as more video feeds are added. The resolution of the session and the recording are lowered as the bandwidth increases.