From the Horus Heresy books (which, remember, went through a retcon with regards to Legion sizes sometime after Fulgrim but before Tales of Heresy), the largest Traitor Legion was the Word Bearers at 150,000 prior to the battle for Calth, and the smallest was the Thousand Sons at 9000 prior to the Burning of Prospero. It is mentioned that the Raven Guard were a relatively small Legion, at 70,000, so we can assume most Legions were 70-120 thousand strong. So I'm going to assume most Traitor Legions were roughly 100,000 in size, but the Alpha Legion and Emperor's Children at 80,000 since they were supposed to be smaller. So that means, and remember that this is an assumption, that the Traitor Legions had about 919,000 Marines going into the Heresy.

Now, when the Thousand Sons fled after the Burning of Prospero, they had just over 1000 Marines. The Word Bearers lost 50,000 on Calth, if we assume only a handful escaped with Kor Phaeron. A third of the World Eaters, Death Guard, Sons of Horus and Emperor's Children died in the Isstvan battles. If we assume that losses were relatively small/adequately replaced by the time of the invasion of Terra, then we are left with the figure of 659,800 Marines at the time of the invasion.

Now, here's where it gets tricky. Right now, the Horus Heresy series, which is essentially the main source of info regarding Legion sizes during this time, is still in the first half/middle of the seven year Heresy. According to old fluff, which is partially contradicted by Aaron Dembski-Bowden's Soul Hunter series, the Alpha Legion (delaying the Space Wolves in the Chondax Sector) and the Night Lords (fighting the Dark Angels in the Thramas Sector) were not present during the invasion, so we can assume their numbers stayed constant. The Traitors would then have descended on Terra with 479,800 Marines, and as they lost, I will assume they would have lost roughly a third of their numbers (this is being conservatives, considering they had the upper hand until Horus bit the bucket). That means that post-Terra invasion, the Traitor Legion would have had 496,700 Marines.

The Scouring would have followed, which is now apparently 200 years long. Assuming another loss rate of a third over that time (remember that once a Legion's homeworld was destroyed, most of them would lose new recruits and therefore be largely unable to sustain even small casualty rates), we get 327,800 Traitor Marines running into the Eye of Terror.

So there we have it, 327,800 Traitor Marines from the initial Legion number of 919,000, a 64.4% loss rate. Assuming that losses are replaced by new recruits (which have been shown) or by renegade Marines, and remembering that most Traitor Marines are now raiders rather than conquerers and defenders, so losses are typically smaller outside of the major Traitor Crusades, that's still a pretty hefty number.

And once again, these are all assumptions based on a limited number of facts drawn from Black Library sources, not Games Workshop codexes or anything.