There only 13 mother *** Super Star Destroyers! This is unexceptionable! Remember Han Solo in ROTJ comments on Vaders command ship too Luke when they are in the shuttle, "Don't worry theirs lots of Command ships..." 13 isn't a lot.
It's more than the number of ships unambiguously identified as Executor-class in the old EU. 20-odd was the estimated total number in The Essential Guide to Warfare - to allow for new stories to add some.
There were a number of other Star Dreadnoughts and Battlecruisers above 4 and 8 km in length to very between Command ships and the Kuat/Fondor's own personal Defense fleet battleships. The reason why the Empire finally lost so badly (besides plot favoritism) is because the Emperor shot himself in the foot with errant Super weapons during Operation Shadowhand, as well as his massive hardon for finding a replacement for Vader. A massive portion of the Imperial fleet was above Byss when the Gravity Gun missfired from the Eclipse ramming it.
Regardless, the Imperial Fleet after Endor in the new canon is severely depleted and a lot smaller. There is a lot more error to go on with mass defections and civil uprising. The timeframe for the Imperial Surrender is a lot shorter, more poorly written, and more nuanced than the Pellaeon–Gavrisom Treaty, as well as more negatively impacting (Pallaeon never surrendered). It was probably written intentionally to justify the FO buildup much like the German humiliation and anger that led to another disaster after the first WW.