There's a difference though between what is actually sold to consumers and what the placement orders are. Most sales reporting figures are round numbers because they reflect what the Purchase Order was from vendors to Rockstar. This includes everything from big box retailers to mom-and-pop outfits.
In terms of what we should expect, I think it's important to remember that RDR is a genre game, which is going to restrict it's market to a degree to niche enthusiasts and R* fans. I don't expect the number of units to match pace with GTA V at the end of the first year, but it's going to depend on what the holiday release competition is. Having a launch date at the end of October is genius really, because you have the hardcore fans snapping up copies immediately, and within a month, you have another boon during Black Friday events as people look to pick up the "hottest" games for their significant others or kids as the Holidays approach.
The online modes are going to have a big say in how large the user base for this game ends up being. Take away GTA V's online modes, and I believe you could easily slash the final sales figures in half. As important as the story and single player modes are, a robust online component enhances replay value in a way that is impossible to ignore. When folks are being asked to spend more than $60 on a game, there HAS to be the perception of value or else the base won't be sustainable.
When you look at a company like Ubisoft, who had a string of launch failures (The Division, For Honor) affect later titles (Ghost Recon Wildlands), strictly based off the state of the game at launch, R*'s launch of RDR2 needs to be fully formed and robust for both single and multiplayer modes, or else they'll be suffering a setback in advance of the biggest shopping day of the year.