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:
Tomatoes are considered to be an excellent cure for hair loss. It is believed that application of tomato pulp on scalp can prevent hair loss. Tomatoes grown for canning and sauces are often elongated, 7 9 cm (3 4 in) long and 4 5 cm (1.6 2.0 in) diameter; they are known as plum tomatoes, and have a lower water content. Roma type tomatoes are important cultivars in the Sacramento Valley. Quite a few seed merchants and banks provide a large selection of heirloom seeds. The definition of an heirloom tomato is vague, but unlike commercial hybrids, all are self pollinators that have bred true for 40 years or more