Personally, I'd go with 5 if they're introduced at the same time.
Well, actually, I'd go with one and a few other smaller upper jaw bichirs such as senegalus (and maybe some variants), members of the palmas complex... etc...
Currently the tanks has 1 8" delhezi, 2 tiny 2" delhezi, 1 5" albino senegal, and soon another 4" endi endi. I think I should stop adding more bichirs so I don't over crowd the tank.