Just occurred to me that there might not be one
Saiyan saga has kaioken which isn't technically a form but may as well be Frieza saga has super Saiyan, cell saga has super Saiyan grades and super Saiyan 2, buu has super Saiyan 3, bog has god, then blue, blue evolved and ui
Am I missing one?
No, except for maybe the Trunks from the future arch, super sayian 2 was kinda new at the moment, but still already seen on the main series. (edit, I think on that arch Trunks doesn't get SS 2 but SS1. I don't remember well)
Dragon Ball has the curse of needed to introduce new forms and raise the power level in order to continue the story.