It's hardcoded and impossible to change. What you can change is the number of brigades per division, which is controlled by the variable BRIGADES_IN_DIVISION in defines.lua. What you can also do is add more than 5 divisions to a corps, more than 5 corps per army, etc. in the OOB files. Keep in mind that the OOB files only work when the game starts or if you fire an event that loads them, and AI-controlled countries and AI-controlled HQs with permission to reorganize their subordinates will rearrange what you set up.