Very often I hear something like: "I couldn't find what I was looking for so a staff helped me." 'a staff' is incorrect because "staff" is a group noun. It means ALL the people who work somewhere.
"Staff at the Imperial Hotel are very professional."
If you want to count them one by one, please use a counting word. Almost always we use
'member'. "I couldn't find what I was looking for so a staff member helped me." Or you can use other nouns that are the names of the things in a group: staff = president, vice president, department manager, section manager, secretary, office workers, cleaners --> 10 staff members
furniture = shelf/shelves, desks, chairs --> 10 pieces of furniture money = notes, coins work = jobs, projects, tasks homework = projects, essays, reports It is okay to use plural for the things in the group, so "s" is okay. e.g. coins, jobs, essays
But please don't use plural for the group nouns: furnitures