The good news is that, if you've read all of his adult solo fantasy novels to date, you're already six books into it. And 'series' is probably the wrong word, a more accurate term would be 'setting'.
It's been an open secret for a while that Sanderson's fantasy novels share a common background setting and mythology, the 'Cosmere'. In his first five novels - Elantris, Warbreaker and the Mistborn trilogy - this took the form of a couple of easter eggs. Most notably, a character called Hoid plays a minor, background role in all five books, apparently observing events with interest.
In The Way of Kings this background suddenly became more important to the plot: Hoid (aka 'Wit') now has a brief POV section and plays a larger role in events. We also meet three other people who can travel between the worlds, two of whom we've met before (one in Elantris and one in Mistborn), who are apparently trying to track Hoid down. Hoid sends a letter to the organisation that sent them (an organisation called the 'Seventeenth Shard') expressing irritation with this move, a letter that appears throughout the second part of the novel. Ultimately, it is clear that the ten-volume Stormlight Archive series will expand on the Cosmere and the linked setting of Sanderson's fiction.
At the moment these works exist in the Cosmere setting:
- Elantris (2005)
- Mistborn: The Final Empire (2006)
- Mistborn: The Well of Ascension (2007)
- Mistborn: The Hero of Ages (2008)
- Warbreaker (2009)
- The Stormlight Archive: The Way of Kings (2010)
- Mistborn: The Alloy of Law (2011)
Sanderson plans to write the following books in the setting (and in some cases has already written very early drafts):
- The Stormlight Archive books 2-10
- Several further Mistborn side-novels featuring Wax and Wayne
- The Mistborn II trilogy
- The Mistborn III trilogy
- Warbreaker II: Nightblood
- Elantris II
- The Dragonsteel series (seven volumes, first one is The Liar of Partinel)
- White Sand and at least one sequel
- The Silence Divine
- Aether of Night
That accounts for 28 further books in the setting. Combined with the six already published, that's 34 books with two left unaccounted so far (recalling that Alloy of Law and its forthcoming sequels are not part of the count, being new inventions). There may be a further Elantris sequel, and Brandon has also suggested that there may be a book called Hoid which tells the story of the titular character in much clearer detail (though apparently the Dragonsteel sequence will reveal a lot more about the underlying mythology and unifying points of the various books and sub-settings).
So far the Cosmere has been something that close readers have picked up on, but casual readers are probably totally ignorant of it. There are shades here of Stephen King's unified supernatural mythology: readers can read The Stand and Eyes of the Dragon with no real clue who Randall Flagg is, but then in The Dark Tower series more information is revealed about him and a grander masterplan can be discerned. This doesn't prevent the books being enjoyed individually but does reward readers who've been looking at things carefully.
Hopefully, by 2035 or thereabouts (when no doubt ebooks will be inscribed directly into our brains with lasers or something), we can look back and see how successful Sanderson was in pulling off the project. But it's certainly an ambitious - even grandiose - idea and it will be fascinating to see it develop in the years to come.