1. a mod with 1 secondary stat at lvl 1 will improve that secondary stat once (Guaranteed!)
2. a mod with 2 secondary stats at lvl 1 will have 2 improvements at lvl 4 and 6. RNG decides which of the 2 base stats will improve.
3. a mod with 3 secondary stats at lvl 1 will have 3 improvements at lvl 4, 6 and 9. RNG decides which of the 3 base stats will improve.
4. a mod with 4 secondary stats at lvl 1 will have 4 improvements at lvl 4, 6, 9 and 12. RNG decides which of the 4 base stats will improve.
https://forums.galaxy-of-heroes.starwars.ea.com/discussion/59755/massive-mods-guide/p1