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If I am not wrong actually your are asking about the number of people attending the class that what I have figured out from your question. How ever it is very important for the trainer him self to be prepared for the topic he is going to deliver particullarly in HSE issues where life of people depending in what your going to deliver so if your not mastering your topic do not do it
As for the number of people (from my own point of view) the class room should not exceed 20 persons maximum to allow the trainer controlling the trainees with good eye contact with them and make sure every one figures out at least 80 to 85% from the topic and again (sorry for the repitition) life of people depending in what your are going to tell in HSE classes