I would like a formula to Rank performers from 1-5, example:

In Column A from A1 down to A100 I have a list of Employees by Name
In Column B from B1 down to B100 I list their employee ID#
In Column C from C1 Down to C100 I have their total amount of sales last
month

I would like to rank each employee on a scale from 1-5 (5 is the highest)
based on their performance. ( i.e. The top 20 should get a 5)

I can't figure this out,
Can you please help?