For one class
Go to Students > Attendance > Registers By Date. Select the register and click Lesson Overview.
In the Students table on the Lesson Dashboard, you can see how many students are in the lesson.
For all classes, or certain subject classes
Go to Students > Enrolment > Academic > Courses > Table View.
On this page, you can view how many students are studying each bottom-level course, then export this to Excel or Live feed to find the total studying each subject.
You might only see students enrolled in the bottom-level course (which is what we recommend) rather than the subject, depending on how your school manages enrolments. The bottom-level courses (the ones with the longest name) determines which students are in which registers so this is the important bit showing how many students are in each class.
Why is the number of students showing as 0?
Please see this article: Why is the number of students in the class showing as 0?