Staff is a complicated word in English. This post demonstrates the various uses of staff, and indicates which uses are best for IELTS and also which uses are best avoided.
If you use staff as a singular countable noun, then you are probably not referring to someone who is a worker.
Used as a singular countable noun, a staff is a kind of stick with certain features and functions:
Used as a plural countable noun with 's', staffs means more than one staff (= more than one wooden stick!):
You are extremely unlikely to need this meaning in the IELTS test!
Used as a plural countable noun without 's', staff refers to people who work for a particular organisation, or within a particular department:
Notice the subject verb/agreement:
In the context of IELTS writing, you will probably be talking about people working for a particular company or organisation, and so you will probably never need to write 'a staff' or 'staffs'.
This is extremely uncommon. It refers to a component of a company or organisation, just as you might refer to other components: accommodation, infrastructure, production, shipping, etc.
Notice the subject verb/agreement:
Again, this use of staff (uncountable) is extremely rare and I suggest not using it in IELTS.
In IELTS speaking and writing use staff plural countable without 's':
If you really must use a plural countable noun with 's', you can do this: