How many SDF-#'s are there?
The Robotech Wikia has a list for SDF-1, SDF-2, SDF-3, SDF-4 and SDF-7. It includes a listing for SDF-M, but I don't think that was in the TV series (it's mentioned in the Robotech comics). Wikipedia also has a list of Robotech vehicles that has a lot of detailed information of 1-4.


SDF-1: Robotech TV series
SDF-2 Megalord: Robotech TV series (starting from episode 26)
SDF-3 Pioneer: First appeared in "Crystal Dreams" promo and in the Robotech II: The Sentinels series
SDF-4 Izumo/Liberator: Last episode of the original series
SDF-7: Robotech II: The Sentinels novels 
SDF-M: Invid War: Aftermath comics


The SDF-7 was a Horizon Class T ship, so it's not like SDF-1.