Beagleboard.org say they have made, distributed and sold 200 units/week pretty consistently since launch, that works out to roughly 10k/year. Its really hard to guess, but if R.Pi ended up shifting 10x that, I wouldn't be surprised.
There are so many factors to take into account. Will the prices of particular parts double or half in price during the year? How confident are the team that there aren't any hardware bugs in the production boards?
I guess I would probably try to have 10k available for launch; a manufacturing partner that is equally happy making 100/week as they are 400/week; and attempt to place an up-front order for, say, another 10k to be delivered over the year but with the flexibility to modify the design at any time.
I'd go for all model B for the first 10k, with a plan to stockpile the model A for a January or February launch.
I dunno - I'm just glad I'm not the one making the decision