H = Number of households
All members of P will require information from the site.
Open enrollment for most company health plans is from Oct 1 - Oct 15.
Most members of P who need to compare to company health plans will need to access the site between when it opens - Oct 1 - and the deadline for open enrollment in their company plan - Oct 15.
Assuming 80% of P is in the above category:
L = (P - H) * 0.8
Using data from census.gov:
L= (240133500-114761359) *.8
Multiple by number of clicks/page loads to accomplish the basic tasks and you have a number of approximate hits. From what I have seen of the site, that is approximately 20 page loads.
So 2,005,954,240 hits / 15 days / 24 hours per day / 60 minutes per hour / 60 seconds per minute = ~1548 hits per second minimum capacity required.
If we further assume that the initial surge will be more than 1/15 of the population, then that number can be scaled appropriately.
It's not black magic.