The story Odds attempts to paint these figures as Republic disinformation to sell the war to the public, and that the actual strength of the Confederate Droid Army is several hundred million. However the “quintillions” statements can be taken as accurate without the “bias” or “disinformation” that may be in other works, as the Dorling Kindersley books are written as outside context fact books – they are not attributed to being written by a “scribe” or a committee of “scholars” or “historians” like West End Games books and Essential Guide books are. Further, it is unlikely in the extreme that the Supreme Commander of a military force would be ignorant of the forces he commands. Republic disinformation would not change General Grievous’ own troop counts.
The story does not necessarily contradict the rest of canon however (and thus it can remain canon instead of being classified as “Infinities”). The Droid Army strength in it is derived from the production rates at a few factories on only forty-seven planets out of thousands. But this is the number produced in the past year, and ignores that the various interests that came together to form the Confederacy were building up their armies for at least a decade prior to the Clone Wars - The Phantom Menace indicates that over five billion droids were deployed to Naboo alone. It is possible that the factories infiltrated by the commandos are dedicated to producing mines, thus explaining the discrepancy. That General Zey did not correct Sergeant Skirata could be explained by the fact that the Sergeant has previously demonstrated he is unreliable and willfully disobeys lawful orders that will hasten the end of the war out of personal interests. It is clear that the general is lying to the sergeant – the claim that deploying several million to a sector is excessive force flies in the face of the fact that several million were deployed to Munnilinst alone (The New Essential Chronology) and the Republic was performing Base Delta Zero operations on worlds. (Attack of the Clones Incredible Cross Sections; Republic Commando: Triple Zero; Labyrinth of Evil) Sergeant Skirata possesses extremely poor intelligence on other topics e.g. the total strength of the Grand Army of the Republic, the strength of the Grand Army of the Republic relative to the Confederate Droid Army, the abilities of Jedi (Republic Commando: Triple Zero), so other evidence backs this interpretation. It would also be a wise decision on the part of General Zey, as the fact that a general would choose to disregard the advice of a mere sergeant prompted Sergeant Skirata accelerated his plans to desert. Thus the upper limit of Confederate strength stands in the “quintillions”. And even if those sources were rejected, the observed production rates of factory worlds such as Hypori mean that the Confederate Army cannot have a strength lower then several trillion (10^12) droids.
However, “quintillions” is not the set value, but the strength of the droid forces at their peak shortly before the first Empire Day. Initially, the Grand Army of the Republic outnumbered the forces of the Confederacy (CIS Shadowfeed 14:2:14, "Dooku Addresses 'Spurious' Republic Reports" SW Insider 65, page 75), not surpassing them until late in the war (Dark Rendezvous). The Confederacy was forced to generate superior logistics to blunt any attempts by Republic forces to advance (Shatterpoint), and extensively mine vast tracts of space to hamper Republic mobility (CIS Shadowfeed 14:7:01, article "Kuat-Neimoidian Arms Race Escalates" SW Insider 68, page 69). This was aided by the pre-staging of their forces; prior to hostilities their armies were already stationed across the galaxy (Star Wars Databank – Confederacy of Independent Systems; Shatterpoint). The restriction in the Republic’s ability to sweep through and render them unable to bring power to a point is apparently how the Sith Lords maintained the firepower parity through the war (Revenge of the Sith Visual Dictionary). This would give them time to bring to bare the great advantage of an artificial army – its faster exponential growth rate, which is detailed in the Production section. So while the peak strength of the Confederate army is known and its growth pattern, what was its initial strength? How many droids did they begin with?
The first visible source for this is the blockade and subsequent invasion of Naboo. (The Phantom Menace) Examining the scope of the blockade and the forces deployed means that the military of a single one of the signatories of the Confederacy is large enough that they can send over 5 billion troops to a single peaceful world for “almost a month” (The Phantom Menace novel) and it will have no impact on their other shipping and security operations. This value serves as an absolute lower limit on the size of the Confederate Droid Army, as it discounts further production and the forces the other seven members of the Separatist Council had at their disposal.
That the Trade Federation would commit so many troops to invade a peaceful planet like Naboo fits with later Confederate deployments and the stated peak number of troops. The Confederacy committed millions of droids to single operations as a matter of routine (Star Wars: Complete Locations; Jedi Trial; Clone Wars Chapter 22; Revenge of the Sith; Clone Wars: When They Were Brothers), and individual ships can carry anywhere from tens of thousands to over a million battle droids. (Revenge of the Sith Incredible Cross Sections) Together this speaks volumes about the size of their army. One does not commit significant percentages of your forces to small operations, particularly when one is a brilliant a military strategist as General Grievous is.