Enter "1". Single with one job, no dependents, and not a dependent on any one else's tax return, you'd enter a "2".
Unless you are so bad with your finances that you couldn't pay any taxes due in April if you owed, then you might want to enter a "1" or a "0".
In general you want to take and use all of your available deductions and exemptions and credits. No sense overpaying on your taxes.
Also, remember that if you overdeduct over the course of the year and thus will get a big refund check, that you have effectively GIVEN a free loan to the government for that year.