The ACS Atlas would give a clue as to how many would be needed; approximately 2000 zones in the Olson format would be needed to match what is in the ACS atlas and it isn't really "complete".  The mapping of time zones appears mathematically to be like a logarithmic spike with one end toward relative uniformity (around 400 zones on the low end after 1970) and the other end to where each clock has its own unique time zone history (aka LMT).  It is a matter of where you want to draw the line.  The farther in the past you go toward the date of onset of standard time or LMT the more chaotic it gets (to the point where it simply would be impossible to have certainty for many locations).

 

Due to the fact that most people reside in cities, ACS went about as far as reasonably possible given the limited manpower.  It was no doubt an extremely tedious task, but I do wish that they had made their research methods and criteria for selecting daylight transitions public.  As the data currently stand, my confidence in transition times for DST for dates in the first half of the 20th century is shaky at best.  I only feel relatively confident if it is near the dead of winter or the height of summer for this period in history.