Can anyone explain what kind of viewer numbers the show will "need"? What's considered normal, bad and very good?
If ratings stay as they are, HBO will be very happy. If they drop a bit and then build again by the end of this season and into the next, that will be reasonable. If they drop and stay at a notably lower level throughout the two seasons, HBO might have to scrutinise things more carefully.
However, HBO have made back a bit under 50% of the cost of Season 1 already due to international sales. Assuming those sales are contracted for multiple years, that makes later seasons more attractive: instead of having to spend $50 million out of their own warchest to fund Season 3, for example, they only need $25 million since the rest is taken care of through international sales. I imagine that's in the ballpark of what
True Blood costs per year, which is a relatiely cheap show by HBO standards. Don't get me wrong, HBO can't keep GoT going for multiple seasons with less than 1 million viewers like
The Wire, but they certainly don't need the massive ratings that it might look like from the budget.
DVD sales will also play a significant role in this. Because of the show's profile with non-HBO subscribers, expect the DVD sales to be very healthy indeed (though I suspect a fair few will have seen the series already through various means, but a great DVD set with excellent extras will still sell well). There'll also be iTunes sales to consider once the series gets on there.
Overall, the series has started well and can withstand a modest (but not too severe) drop in ratings as long as it climbs again (which is what happened to
True Blood).