The wiki page for the Ed, citing articles dated September 2015 from the NY Times and New Jersey Monthly, says the seating for Colbert's Late Show is 370. The page also lists the capacity of the building as 457, but I'm sure that's accounting for the folks onstage and backstage and complies with the relevant rules from the fire marshal or similar official.
The page also says that the orchestra level was originally raked (angled for better views), but then leveled in later renovations. Even with smaller seats back in the day, I'm having a hard time seeing how 1200 seats would have fit. But I'm not in the event business.
If the Ed gets converted back into a theatrical space, they'll need to add at least 130 seats to be an official 'Broadway' theater, even being on Broadway. The 500 seat rule wasn't in place when the Ed (then the Hammerstein) was a stage theater, but I suspect it had something closer to 1200 seats then anyway.
David