See picture. The number of notifications flows into the “Stack Exchange” text when I have > 10 notifications.
Furthermore, aligning the baseline of the number and “Stack Exchange” looks kind of weird. It would look better to center the red blob vertically (but that’s unrelated to the number of digits).
EDIT (not by OP):
Looks even worse with a three-digit number:
"tackExchange: The Internet's #1 pushpin enthusiast site! Buy/Sell/Trade!"