If you conduct a training ( say Confined Space Entry Awareness) for 50 people,would the outcome of the training really benefit the delegates?

If you start with a question that you are guessing or not sure that your training session would be beneficial, why you start it from the beginning and bother yourself to conduct it? Confined space Entry is very important issue and there are a lot of mistakes that happens there.
Awareness to that matter is required especially with people that cannot handle that situation physically and psychologically.
Defining the requirements and the preparations for such places is important including any obstacles that might exist in order to prevent/minimize (being proactive) injuries and mistakes.
Hope that satisfies your query, kind regards and good luck

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