attendee (of a lecture); student (in a class/course); participant
Refers to a person who attends a lecture, course, or training session. More specific than 参加者 (participant in general events).
There are 20 participants in this course.
Attendees should read the textbook in advance.
学生 is a student in an educational institution, whereas 受講者 is anyone taking a specific course, including non-students.
total number of trainees
Compound of 受講 (じゅこう, 'attending a lecture') + 者 (しゃ, 'person').