How many articles should a PhD candidate in applied linguistics /humanities optimally publish during their PhD?
To add to GrotesqueSI's answer, while it might be meaningless to associate an exact number to "hard-working" students, as a lot of other factors matter, I am guessing you are asking this question as a way of setting a goal for yourself (or assessing your work-in-progress).

A simple solution might be to pick a sample of 10-20 alumni that you consider to be successful or "hard-working," or at least in a place career-wise that you hope to end up one day. Take a look at their publication record from graduate school: what is the average number of papers? Is there a large spread, or do they all exceed some minimum number? For those who succeeded with few publications, were they particularly influential publications?

This can help you to find the answer that is personally most useful to you.