In the case of a fire where all apartments are occupied (each by two people on average) in your SF apartment, you have in theory 40 people/2 stairs= 20 people e

In the case of a fire where all apartments are occupied (each by two people on average) in your SF apartment, you have in theory 40 people/2 stairs= 20 people each (though it is more likely that there is a "main" stairwell, so that will take - say - 28 people and the other one 12. In a small multi-apartment block, up to 10-12 apartments (5/6 on 2 floors or 2/3 on 4-5 floors) the single stairs/fire exit will be used by the same amount of people or less. If you prefer, if you graph it, the function of number of stairs per block depends on number of apartments and the points are: 1=1 2=2 3=2 4=2 5=2 ... what the author is saying is that the step at 2 apartments is too low, compared to what are the norms in other countries and that these countries have not worse records, statistically, for deaths in fires. This latter is debatable, as there are so many factors involved such as efficiency of rescue/fire services, type of construction (materials used), height of rooms, sizes of windows, number of floors of the "average" building in any given country (and even local situations, city by city) that it is difficult to do a sensible comparison.