Well, if all the inquisitors in the Imperium ascended to daemonhood overnight, they wouldn't have
anybody to pull from; even their own household troops would turn on them. Of course, that's not how it would work; it's not like the Chaos gods just hand out daemonhood willy nilly. There are Chaos legionnaires who have served the dark gods for 10,000 years who haven't hit daemonhood yet.
A much more plausible scenario is that an inquisitor falls to Chaos over a period of time, quite possibly by accident, and take his household troops with him. In that sort of scenario it's highly unlikely that any troops from his Ordo's Chamber Militant would follow him.
You could spin it a couple of different ways. If you wanted to be fairly literal about it, even an Apocalypse game probably represents a very small battle. For instance, the 12th Tallarn Armored regiment, a fairly typical Imperial Guard tank regiment, at the time of the Taros campaign, fielded
one hundred and fifty nine Leman Russes, eleven Baneblades, ninety Sentinels, one hundred and six Chimeras, forty-eight Basilisks, twenty Hydras, and seventy-nine Salamanders, plus support vehicles. One regiment - one of
two that fought in the Taros invasion, which was a
small invasion by Imperial Guard standards. Since even the largest Apocalypse battle you're likely to be able to play is a tiny, tiny engagement by 40K standards, you have the luxury of making your campaign be about a single rogue inquisitor if you want it to be.
In fact, let's say you don't want to be quite that literal about it, but still want to run a campaign wherein marines from several chapters invade a planet and do battle with one or more inquisitorial players. That can
still be quite a small number of inquisitors - as small as one - fluff-wise. Keep in mind the following:
- Even a single rogue inquisitor could induct into his service as many Guardsmen and PDF troopers as he could lay his hands on. A handful of rogue inquisitors could easily possibly command tens of thousands of soldiers.
- While the Chambers Militant would be unlikely to fight for a rogue inquisitor, it's not like rogue inquisitors go around with a flashing sign over their head that says "Rogue." Very often in 40K it's not clear who's rogue and who isn't. If you want, say, sisters of battle to fight with your inquisitor(s) in your campaign, it's easy enough to say that they simply disagree with the invading marines that the inquisitor has done anything wrong, and are going to fight to protect him or her.
- A codex marine chapter has very limited manpower. In fact, merely crewing every vehicle in the chapter's armory will take most of the Sixth and Seventh Companies (organizationally, the reserve companies are really there to support the battle companies, which means that a marine chapter really has basically five "fighting" companies, plus support). As a result, chapter masters tend to be very cagey with their use of men. I won't say that they're cowards (unless I'm having a *****-fest with Melissia about how much I love to hate marines ;)), but they're very aware of how few men they can afford to lose. If you ask a chapter master to do a job that will take 1,000 marines to complete, he'll most likely tell you no - because that would mean putting his entire chapter at risk of annihilation, and very, very few missions are worth that to a chapter master. Instead, he'd try to get a coalition of chapters together, each of which only contributes a few hundred men, so that no one chapter is at risk of getting wiped out just because the mission goes south. It's not like the Ultramarines allocate all ten companies to a mission, then ask the Imperial Fists to allocate all ten of their companies to the mission, and then ask the Blood Angels to allocate all ten of their companies. If a mission requires that many marines, the Ultramarines would try to get, say, fifteen chapters each to commit two companies.
What this means is that you can have a reasonable multi-chapter marine force in your campaign without the mission being so big that it requires the full resources of multiple marine chapters.
Helpful?