Now this is probably a surprise to many since I always see thousands of comments like the following(10k only):
@Sarfraz, @balpha just told you they did in the first comment he wrote. Also, allowing only one Stack Overflow user to be logged in per IP address would be problematic for networks with shared IPs.
What's surprising is that apparently the notification will trigger only for Sarfraz and not for balpha. As you can see from the blog post:
Only one person can be replied to at a time in a comment. The first one in the string wins.
I propose to increase the limit to let's say 3 or 5 people; it won't hurt (never seen that feature being abused) and especially many people believe it already works that way.