noun
BAD : The assembly and packing department has 50 staffs.
GOOD : The assembly and packing department has 50 staff.
GOOD : The assembly and packing department has a staff of 50.
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The noun staff refers to a whole group of people, (NOT a single person): '(The) staff at the Central Hospital have rejected the new pay offer.' 'Several senior members of staff have opted for early retirement.'