I would make a basic assumption that most users have either at minimum two IP addresses or three they use for HN. One would be work, another home and at the minimum one would be mobile although I think mobile IP changes as you travel if I am not mistaken and not guaranteed to be static. So potentially more than three but these could be shared among HN users so maybe 2 at a minimum at the very least.

They do change, and quite a lot, which means users often have many mobile IP addresses. However, those same IP addresses often show up for many users, so it's hard to know how the counts shake out.

That'll be me... Daily one hour train commute, and my IP often changes a number of times as I switch from tower to tower, as well as drop out due to no signal in places.

I also switch between networks as my mobile GSM and portable wifi are different providers.