On the left is a credential with the signatures of the County Clerk and the Chairman of the Democratic Party. You can make as many copies of this credential as needed. You will need a copy for each precinct you visit, each time you enter the polling place. If you plan to monitor 5 precincts, and you expect to visit each precinct twice during the day, you would need at least 10 copies of this credential. On the right is a sample credential with the blanks filled in to use a a guide.
Below is the official Guide for Pollwatchers, which explains the duties and responsibilities of this position.