How many seats does he foresee the Republicans winning without much challenge?
He isn't dealing with the safe seats. His focus is on the competitive seats of which he deals with 66.
In another article, Rothenberg asserts that it's a pipe dream for Democrats to take back the house and expects the result to be basically a wash, the result being anywhere from a +1 gain for the GOP to a +6 gain for the Rats.
Dozens of seats in GOP leaning districts that were represented by Rats prior to 2010 (think Blue Dog) came back to the GOP. So there aren't a lot of seats left that should be GOP, but aren't. In addition, there were quite a few Dem leaning districts that went GOP. Those are the ones that are the most precarious and many of those will probably return to Dems.
In states under GOP control, like PA and NC, the redistricting favored the GOP. In other states, like IL and CA, the GOP got further neutered in the redistricting process, although in CA the result wasn't as bad for the GOP as many expected.