Go to site below, click on KStN and then on Artillerie and you will find a lot of TOEs for Stug units.
Generally in a StuG Brigade or Abteilung there were 21 StuGs and 9 StuHs authorizes.
Brigade or abteilung stab with 1 StuG = 1
Battery stab with 1 StuG x 3 batteries = 3
each battery with 6 StuGs and 3 StuHs x 3 batteries = 18 StuGs and 9 StuHs
in other words each battery had 2 platoons each with 3 StuG and 1 platoon with 3 StuHs