The fluff came out officially at the same time as the rules, in the Rogue Trader (1st edition) rulebook. But the modern fluff is completely different from what it was originally. In the Rogue Trader rulebook, there was no such thing as "chaos daemons", the imperium ruled the galaxy, there was a striking (in hindsight) complete lack of grimdark, and questionably sapient monkeys were a playable race. Since then, the fluff of space marines has evolved from "Highly trained warriors with some physical enhancement and good equipment" to "Badass supermen who can shoot far better than any human, take bolter rounds and live, crush men with their bare hands, and wear tanks". A huge driving factor in that was to sell models, so much so that the fluff eventually became completely disproportionate with the actual rules.
In contrast with the fluff, the rules for space marines have stayed relatively stable. They have been, in my opinion, the base upon which the other armies are balanced. Because of this, the rules for space marines, as i mentioned before, have lost touch completely with the fluff. For this reason, I think it's safe to conclude that the fluff and the rules are completely incomparable. So Platypus5, to answer your question, in the fluff, a crapload of guardsmen. On the table, eh, 4? Depends wildly on the circumstances for both.
I do disagree though that this argument "transcends fluff and becomes an argument over the nature of 40k itself". What is "the nature of 40k"? If it's not fluff, is it rules? Because in my opinion, fluff is just as much a part of the game as rules. Although more people play the game for the game and not the fluff, without the fluff, there would be no game.