Webb and Chafee are out of the running. O'Malley will likely hang in through the Iowa and New Hampshire debates. That leaves mostly Clinton and Sanders to duke it out five more times, with a possible sixth encounter at the the MoveOn forum. There is some pressure from the non-Clinton wing to expand the number of debates, but it is a little hard to imagine that the Dems could gain enough of a national media audience to support additional debates unless they moved to the carnival freak show format that the Republicans have opted for.
Clinton's debate objective will be to undercut Sanders by cherry-picking issues that help her to shore up her credentials with the party's left wing. She will continue to hammer him on gun control until -- if and when -- she gets the nomination, and then she'll evolve to a more moderate position. Clinton will likely make references to the Supreme Court's money in politics position, but she'll say nothing which will seriously irritate her big bucks backers. Neither Clinton nor Sanders will have anything substantive to say about the Israel-Palestine confrontation.
Sanders' task now is to counter the Clinton/big-media story line that the race is already over before the first primaries. His email campaign is presently focused on showing his consistent civil rights stance over a period of many years with an eye toward gaining more traction with blacks, hispanics and the LGBT communities. Those efforts are no doubt based on accurate perceptions of the lay of the land, but Sanders will also need to come up with better answers than he had available in the first debate regarding his economic analysis and specific and credible remedies. To is credit, Sanders is the only one in the race who consistently alludes to the fact that achieving the presidency does not translate to leading the way to real changes without the support of millions of Americans -- that is, the achievement of a progressive majority in Congress.