Charles Ellicott Commentary Numbers 35:30

"Whoso killeth any person, the murderer shall be slain at the mouth of witnesses: but one witness shall not testify against any person that he die." — Numbers 35:30 (ASV)

By the mouth of witnesses.— The number of witnesses is not here specified. In Deuteronomy 17:6 it is ordained that the crime of idolatry should be punished with death at the mouth of two witnesses, or of three witnesses; and in Deuteronomy 19:15 it is ordained in general terms that one witness shall not rise up against a man for any iniquity, or for any sin that he sinneth: at the mouth of two witnesses, or at the mouth of three witnesses, shall the matter be established.