If there was really "well over 250 dungeons" you'd be tripping over them. There would be a hole in the ground every 20 yards.
You mean like oblivion?
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Todd did say theres "over 150 dungeons" in the game. 250 is over 150, and oblivion had close to that number I believe. Granted I doubt he'd use "over 150" to describe 250, and instead say "over 200" or something closer. am I suspicious? Yup, but it is within the realm of possibilities seeing as oblivion had that much.
Also, there could easily be over 150 unique dungeons and then another hundred or so generated ones. I'm sure they still have the program used for oblivions dungeons hanging around, and could tweak it for making more dungeons for skyrim.