Wow @Piemax2 how did you figure that out!
It seems to be the number of mods available for each unit. But only if you haven’t opened the unit up in the modifier/customizer screen. As soon as you go to the “Manage Mod” screen and open the unit up the numbers goes away in the unit build queue.
Oddly it doesn’t show up in the list of units in the “Manage Mod” screen, which seems like where it would be most relevant.