the problem isnt about how many dlcs beth releases. It is about whether they are worth the money.
I could have only 1 dlc for $50 but if the dlc is awesome, it is worth it.
Nobody would want to pay $20 for dawnguard if its like that knights of the nine add on...
I liked Knight's of the Nine, but it was only ten bucks. Back then, Bethesda was also experimenting with pricing structures as well. That's why we had the infamous Horse Armor. They didn't really know what was appropriate, and what people would pay for DLC's. Honestly, nobody did, the Xbox360 was literally in its infancy.
Now that they have a much better understanding of how to work DLC into the platforms, costs, revenue potential and other business savvy goodies, They're in a better position to price their DLC appropriately.
For all they learned, and how much larger the install base of Skyrim is, $20.00 for the DLC, when Shivering Isles was $30.00, combined with many other facts (Team Size, Development Cycle, etc) Dawnguard
could be as large, content-wise, as Shivering Isles. I'm mostly saying it's not necessarily telling that it
will be smaller, based on price alone.