Final answer:
As of the end of Season Five of "The Office" there have been four receptionists: Pam Beesly, Ronni, Ryan Howard, and Erin Hannon.
Step-by-step explanation:
By the end of Season Five of "The Office" (US), there have been four different receptionists at Dunder-Mifflin Scranton. Initially, Pam Beesly is the primary receptionist. During her temporary leave from the position, Ronni briefly replaces her.
When Pam moves to sales, Ryan Howard covers the reception desk for a short while after returning to the office post-firing. Finally, Erin Hannon takes over the role and becomes the permanent receptionist by the end of Season Five.