There’s no specific limit as to how many devices you can actively monitor from the teacher console.
Having said that, there is a limit as to how many students you can see on ONE screen at the time.
Monitor & Control
In this mode, you can see up to 36 student devices at one time. This is the optimum number in Grid mode so you can actually see their activity in a small enough grid.
Slider Screen Mode
However, you can setup Slider Mode to see several screens in intervals of for example 5 seconds.
You can do this by clicking the Wrench icon at the top of the Monitor & Control mode and choosing the number of people you’d like to see and the time intervals.