Mishnah tractate Parah is the only source that supplies a count, and it records a dispute over the total rather than a settled figure. No independent documentary, epigraphic or archaeological attestation of any single performance has been identified, so there is nothing outside that one list against which the number can be checked. Nobody holds a defended position on the true count; the record is simply absent.
What would resolve this An independent Second Temple period attestation would settle at least one performance: an ostracon, papyrus or inscription referring to the preparation of the ashes or to the water of lustration, or a securely dated installation on the Mount of Olives identifiable with the rite. Systematic re-reading of the Judaean desert documentary material for purity-water vocabulary is the cheapest place to look.
OpenSecond Temple epigraphy and papyrology, archaeology of Roman JudaeaAnonymous (rabbinic compilers), The Mishnah