actually after a little more work I think I can answer my own question.
Li01_01 in vanilla has 14 factions on base (so it’s possible to have up to 14), however only 3 offer missions.
After a little experimentation I’ve managed to get up to 6 factions offering missions from a single base, after that I get a server crash.
Hope this helps someone, hope my findings match other peoples.
Cheers,
Rik