I'm just wondering of how many publications /articles a phd student in applied linguistics /humanities should optimally publish during the course of their studies? I'm talking about what would considered as an indication of hard work.
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.
I'll be speaking from the "/humanities" side of your question but I am sure this applies elsewhere. There's no answer to your question, I'm afraid. There is no optimal amount of publications and there is no exact number that would indicate "hard work". Ten low-quality publications in non-peer-reviewed journals or books would mean little. One extremely good, very influential publication that makes a significant contribution to your field is likely to be enough to land you your first academic job post PhD. It isn't about numbers, it is about quality.