Has the number of angle of attack sensors been reduced in Boeing 737 Max, in comparison to the older models?
There is already an answer on here about how many the B737 MAX have (the answer is 2), and I've found this Airworthiness Directives which states somewhere far down:


  Do a general visual inspection of the left and right angle of attack (AOA) sensor as applicable, to determine if a certain AOA sensor with a paddle type vane is installed[...]


This brings me to the conclusion that there always have been 2 AOA sensors on the B737, as this Airworthiness Directive is dated to 2012. (To be fair, at least since then there have always been 2 AOA sensors).

As a decrease in the amount of AOA sensors from 1967 to 2012 seems unlikely to me, I'd say there are and always have been only 2 sensors on the B737. Following that train of thoughts, the amount of sensors has thus not changed with the different versions of the B737

Interesting sidenote: The AD is valid not only for B737 but also affects all kinds of 727 and 747 models, which indicates that they also have 2 sensors only, but doesn't necessarily mean that other Boeing aircraft have more!