First, I'll do the fractions, then I'll be working out the numbers between the following results.
Note that this DOES NOT include Megas or Alolan Forms yet(unless you want them)
So, there are currently 802 Pokemon.
In the 1st generation, 20 Fully Evolved Pokemon had higher Special Attack than Attack, excluding Legendaries, Megas, and Alolan Forms
In the 2nd generation, 20 Fully Evolved Pokemon had higher Special Attack than Attack, excluding Legendaries and Megas Megas
In the 3rd generation, 17 Fully Evolved Pokemon had higher Special Attack than Attack, excluding Legendaries and Megas
In the 4th generation, 17 Fully Evolved Pokemon had higher Special Attack than Attack, excluding Legendaries and Megas
In the 5th generation, 23 Fully Evolved Pokemon had higher Special Attack than Attack, excluding Legendaries and Megas
In the 6th generation, 16 Fully Evolved Pokemon had higher Special Attack than Attack, excluding Legendaries and Megas
In the 7th generation, 11 Fully Evolved Pokemon had higher Special Attack than Attack, excluding Legendaries and Ultra Beasts
So, if we compile these results, we will have 124 Fully Evolved Pokemon that have higher Special Attack than Attack.
I did the math, and it turns out that 15.46% of ALL Pokemon have a higher Special Attack stat than Attack.
Let me know if I messed up, which I probably have :P
Hope I helped!