There is no single mechanical rule that always identifies the preferred version. Relevant factors may include superior precision, a pupil's close and prolonged study with a particular teacher, several genuinely independent concordant routes, and indications of a specific mistake.
A greater number of narrators can be persuasive, but numbers alone do not settle every dispute. Several people copying the same mistaken source do not provide fully independent corroboration. A specialist pupil may also preserve their teacher's narration better than others.
The conclusion is a comparative judgment supported by relevant evidence, not merely a famous name, a longer chain, or a raw count of narrators.