At least two, probably three considering Microsoft's deal covers the first two DLCs for Skyrim being exclusive to them for 30 days. Hopefully more than that though.
But no, 1 or 2 hopefully. I'm hoping 'Expansion' not just some house or quest or item. Can't tell how Dawnguard is but hope its roughly same size as SI.
My money is on two, not because of legal things with Microsoft but simply because there have reliably been two add-ons for the past two TES games. Morrowind had Tribunal and Bloodmoon Oblivion had Shivering Isles and whatever that pilgrimage thing was.
I am going to be very disappointed in Bethesda if they decide to take the Fallout 3/NV route and release 5-6, low-content DLC's. What makes the Elder Scrolls series great is that it actually has expansion packs that last for hours upon hours, rather than this micro-purchase crap that's designed to get more money for the developer and publisher over time.
Since they've sold something like ten million copies of Skyrim, I imagine they'll probably try and milk it a bit more than in the past. So I would be guessing at least three.
I reckon 3-5 I mean c'mon it's taken 7.99 months to announce the first, and I'd hope they want to keep the game going with these expansions and offering things, up untill the next major title (fo4 please) So i reckon 3 at least 5 at most!
Three large expansions (with an undetermined number of smaller add-ons, like the mounted combat update I imagine) was the number they gave a while back.