Due to Coronavirus health procedures, students inside classroom have to be distanced $2$ meter from each other, If the classroom dimensions are $(M \times N)$ , What is the maximum number of students can be seated in the classroom?
Is there a general rule for this question?
This is a packing problem. These problems are notoriously difficult, and there is no efficient solution method in general. For example, according to the web site The best known packings of equal circles in a square we might not know (rigorously, at least) the smallest square room that could hold $31$ students.
... unless you want to assume the students are arranged in a rectangular array.