Why are there sometimes numbers in parentheses in the title of a page?
(5) Stack Overflow or like this:
As Mark stated - this is the number of new active questions with activity updated as they happen. We have rolled the realtime active questions update to the entire network except for Stack Overflow where there is no active tab. We are planning on adding updates to other question list pages by tag in the near future so stay tuned.
It denotes how many new posts with activity have surfaced since you last refreshed the page, as described in the MSO question, 1 question with new activity:
Note 1: has since been deployed to a number (if not all) of the sites on the network.