I feel like this requires an assumption that any post you flag is equally likely to be flagged by other users, and at a similar time as you, in other words that your flagging behavior is very similar to others that use this feature.
I guess this might give you a pretty nice upper estimate though. Unless N is very high / very few users use the feature, and you are frequently much later to the scene as the average user of the feature. Then N may be underestimated.
Hacker News is biased so that the threshold for flagging is very low, in order to maximize the effective separation of signal from noise as much as possible (for as broad a definition of "noise" as possible.) If I had to guess it's probably 3, or it's tied to user karma (flags from higher karma accounts count more.)
And certain topics are likely to be flagged by multiple people, this can be assumed based on the topics that people complain aren't "HN worthy." Anything that can be considered politics, for instance, is probably going to get flagged because of the number of people who don't believe political stories of any kind have a place here.
I guess this might give you a pretty nice upper estimate though. Unless N is very high / very few users use the feature, and you are frequently much later to the scene as the average user of the feature. Then N may be underestimated.