Both of these words refer to people who work at a company - but staff is always singular and uncountable - it describes the entire group of workers as one thing. Employees is plural and countable - it describes the collection of individual workers.
The entire staff was happy about the extra day off.
All the employees were happy about the extra day off.
Note that "staff" takes the singular verb was, and "employees" takes the plural verb were.
Whenever you use a specific number, use employees or staff members:
Pineapples that are heavy for their size. While larger pineapples will have a greater proportion of edible flesh, there is usually no difference in quality between a small and large size pineapple. Pineapples should be free of soft spots, bruises and darkened eyes, all of which may indicate that the pineapple is past its prime. Pineapple stops ripening as soon as it is picked, so choose fruit with a fragrant sweet smell at the stem end. Avoid pineapple that smells musty, sour or fermented.