But even if the population remains static, telephone-number demand rises because of pagers, cellphones, second lines, PBX-to-Centrex conversions, CLECs etc. And when demand rises in SxS communities, even ILECs may need new NNXs just to avoid dialing-plan conflicts.
Centerville, Iowa illustrates both of these points.
According to Mark Roberts' original post (ca April 2003), an ESS switch was added for 437 in the late 70s while 856 was still in service. But at the time, 856 was still an SxS switch, and only half of its capacity was used for subscriber numbers (856-2XXX, 856-3XXX,856-6XXX, 856-8XXX, 856-9XXX). The other half was unavailable due to dialing-plan conflicts.
Of course, when 856 was converted to ESS, dialing-plan conflicts disappeared and its entire capacity became available.
Mark also notes that Centerville's population had been "around 6,000 for about the last forty years." According to the 2000 Census, it was8,292 for "Zip Code Tabulation Area 52544." Whether or not a zip code is a good proxy for a telco exchange territory is debatable, but it's the only data available at the moment. Using that data, Centerville's population rose about 37% between "sometime in the 1970s" and 2000.
During that same period, Centerville went from 0.5 NNX (half of 856) to four NXXs (436, 437, 856, 895). Bottom line: between early 1970s and 2000, Centerville's population rose by about 37% but its telephone-numbering capacity rose by 700%.
Taking this analysis in the opposite direction (Ann Arbor) -- before the first ESS NXXs were introduced (761 and 764 in 1964), Ann Arbor had 3.4 NNXs (662, 663, 665, and 40% of 668). It now has 113 NXXs. Ann Arbor has been growing rapidly, but I doubt that it's grown by3,225 percent!