My research indicated that six, is the fewest number of fish recommended for a school of tetras. Apparently, this limits the potential for nipping at other fish as any aggression will be taken out upon their species mates. So, if your tank has room, more than six individuals in the school may be ideal. In my experience, it's worked out well; The Blue School sticks together like glue and ignores Mr. & Mrs. Smith (betta imbellis pair), entirely.