The Space Marine recruitment process is heavily-laden with Plot Armour.
Consider the fact that the M41 Blood Angels are able to maintain the same manpower as the Ultramarines on a nearly constant basis. The Ultramarines should obviously be able to maintain 1,000 battle-brothers (and change); they have nine (if I recall correctly) star systems to recruit from. The Blood Angels, on the other hand, recruit from a single irradiated wasteland of a planet, populated by tribes of appropriately irradiated hunter-gatherers who apparently haven't managed any macro-scale communities, industrial infrastructure, etc., since the Imperium found them.
With that in mind, consider the fact that, back in M31, the Blood Angels Legion (not Chapter) relied on that same planet as its recruiting ground. Sure, Forge World dealt out some serious common sense by making it official that the Legiones Astartes relied on several recruiting worlds, protectorates, etc., for increasing/maintaining their manpower. Still, the Blood Angels must have relied on Baal to provide manpower for more than a 1,000-Space Marine Chapter. There's no way it was just this symbolic headquarters-in-name-only that was only expected to contribute a minimum of recruits.
Hence why I've long advocated the idea that the "Only a few people are ever able to become Space Marines" bit really comes down to many Chapters of the Adeptus Astartes having lost a lot of the Space Marine creation how-to (something that is actually stated outright in the original Index Astartes articles) and/or them making it artificially difficult on themselves.
Meaning, how hard is it to imagine that a cult of superhuman warriors who believe they bear the genes of the demigod sons of the master of the galaxy would needlessly create a incredibly deadly initiation process? Think about it: it sure seems more special when a thousand guys (or whatever) have to die before ONE of them can become a Space Marine, right? Human beings in the Warhammer 40k universe are divorced from reality to begin with, if the fluff quotes from the rulebooks are a valid guide to their thought process and culture. Space Marines are on a whole different level from event that.
The Blood Angels are, in my humble opinion, proof positive of the fact that you don't need a stellar specimen to make a Space Marine. Again, they recruit from irradiated, emaciated wretches whose saving grace is a strong warrior culture and incredible survival skills. Despite the awful effects their planet has on their stature, the gene-seed transforms them into handsome demigod giant vampire warriors. They certainly don't seem to be any weaker than their cousins from other Chapters.
Nor do I think your recruit needs to have incredible mental fortitude or courage. At the end of the day, I'm willing to bet that it's the indoctrination, brainwashing, hypnotic and chemical conditioning, etc., that produces the "And They Shall Know No Fear" mindset. The Adeptus Astartes probably like their candidates to have the patience of a saint and the guts of Sir Galahad to begin with, but, again, I think that's just them fluffing themselves.
Look, at the end of the day, the only thing that probably matters is "genetic purity"... and by this I mean a sufficient absence of mutation and everything more or less in working condition. Feral Worlds probably work so well because the recruiting pool isn't needlessly narrowed down by the sort of hive world pollution that makes most die of five different flavors of premature organ failure. Even then, however, we know that people from some really messed environments are able to pass the sniff test.
To bring this back to the topic... the manpower of the Legions by the time the Siege of Terra kicks off is completely dependent on what the editors and the authors decide is best. The processes that have been described, alluded to, or simply implied make it eminently feasible for the Legions to maintain and even rebuild their manpower post-Isstvan III or Isstvan V*. If the authors want depleted Loyalists and/or Traitors come the 11th hour, though, they'll spell it out just like that.
Hell, don't forget: when "Savage Weapons", The Lion, Prince of Crows, and The Unremembered Empire were written, one of the Big Deals was that the Dark Angels were, like, the one Loyalist Legion still in full strength**. Fast forward to the intro blurb for Gav Thorpe's new Dark Angels Heresy novel, however, and it looks like the First Legion has taken serious losses.
Such is life!
* The Shattered Legions are probably the exception to this because the bottom line is gene-seed availability.
** The White Scars took a beating in Scars; the Space Wolves suffered terribly in Prospero; the Imperial Fists lost the better part of a third of their legion when their Retribution Fleet barely managed to escape back to Terra; the Blood Angels were mauled in Signus; the Iron Hands were one of the Shattered Legions; perhaps a third of the Ultramarines survived Calth; the Salamanders and the Raven Guard were both annihilated almost to extinction.