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:
▶️ Thirty employees received raises.
▶️ Thirty staff members received raises.
▶️ Thirty staff received raises.