Well the first question to ask: Fluff Astartes or Tabletop Astartes?
I would guess fluff marine, without a doubt.
The RPG would be more about "living in the Imperium of the 41st millennum" so it would try to capture the epic feel of the Astartes rather than run a simulation with play balance. Even if you look at the sample marine in Dark Heresy, he was really tough. If you compare him to many aliens in the suppliments, then you can see that ork firepower would just ping off his power armour and an Eldar would only cause damage on a lucky headshot. The Emperor took over the entire Imperium with only a few legions of marines so they need to be extremely powerful.
The flipside is that Space Marines are RARE. A million sounds like alot but spread out through the entire Galaxy they are extremely spread thin. Each one needs to count to make it worthwhile. A planet can usually be assaulted with only a company of Marines. However, they cannot really take casualties so any loss is a blow to them. Deathwatch are even rarer still, with a thousand plus spread over the whole Galaxy they may as well not exist (Douglas Adams had an equation saying that a finite number of people divided by infinite space is zero).
Most of the fluff has hundreds of Marines fighting millions of Orks or billions of tyrannids. I am hoping that they have thought about it and have a system of epic feats which would allow the players to think "yes, with sacrifice we CAN stop this Waaagh"
A few legions...yeah...
But remember, the pre-heresy, back then they weren't broken down into chapters (per se) and there was no Codex Astartes. So the legions consisted of thousands of marines, with no set number other than how many viable canidates could be found.
And honestly, I think that they will aim for some of the more consistent marine fiction from the novels and short stories. Codex fluff rarely specifies numbers, and codex fiction, while being cool, is written to make the army in it sound just super uber awesome, so, yeah, its a bit biased. My guess would be something along the lines of what Graham McNeill, Ben Couter and William King has written, where marines are good, damn good, but not gods of war.