This being 40k, there is no "fixed" or "true" organisation for Commissars in the Imperial Guard, and you will find conflicting information depending on where you look. On the other hand, this allows you to cherrypick what you prefer, or even come up with your own idea.
The following is an excerpt from White Dwarf, just to provide one possible inspiration:
"The Commissar-General is the senior Commissar of the regiment with the longest service and most extensive campaign experience. He assigns Commissars to Imperial Guard officers according to his judgment of the battlefield situation or the character of the Imperial Guard officers in question. Cadet Commissars are allocated to Commissar Training Squads by the Commissar-General of an Imperial Guard regiment. These squad members are identified by a blue uniform trim and Cadet badge.
The Commissar-General assigns one of his Commissars to take the regiment's Cadets and form a tactical unit in its own right, known as the Commissar Training Squad. The unit is made up of one Commissar and nine Cadet Commissars. The Commissar Training Squad accompanies Imperial Guard forces into battle and takes part in some of the fiercest fighting.
The training of a Cadet Commissar has no fixed duration. A Cadet qualifies as a full Commissar on the judgment of the Commissar-General. He will be awarded his Commissar status as soon as he is deemed worthy of it by his actions. This provides great inspiration to the other Cadets on the battlefield. The new Commissar can then be allocated Commissarial duties in his own right. Commissar Training Squads are highly motivated fighting units, respected by all other troops in the Imperial Guard. Any Imperial Guard force accompanied by such squads will consider itself fortunate and probably destined for victory.
When a Commissar decides that a Cadet has failed in his duty, but has not shown cowardice or insubordination, the Cadet is relieved of his position and duties. Commissar Cadets who fail their training can often get a commission in a penal battalion. Others volunteer for service in a Rogue Trader entourage. Sometimes, their destiny will be decided by the Commissar-General or Commissar under whom the ex-Cadet trained."
Based on this, personally I'd say:
- 1 Commissar-General for the Regiment
- "a handful" of Commissars for its Companies, ranging from just a few roaming the troops on an "as needed" basis, to one per every single Company, depending on the Regiment's morale and level of civilisation (Feral Worlders and Hive Gangers might simply require more hands-on oversight than, say, Agri-worlders driven by faith and fear/respect)
- optional: a bunch of young Cadets organised into their own Squad, often acting as Barrier Troops and Firing Squad
Though this is just how I'd do it, mind you.