What should be done if there is no one to establish the proof against someone who has committed an act of disbelief (mukaffir), especially if those who committed this act number in the thousands, and given that you mentioned only a scholar can do that?

The apostasy of a specific person cannot be determined until the Prophetic حجت (proof) has been established against him, and the truth made clear to him. For whoever's Islam is established with certainty, it is not removed by doubt. Declaring a specific individual an apostate is contingent upon him having received the Prophetic حجت (proof), the defiance of which constitutes apostasy.

The Quran and Sunnah indicate that Allah does not punish anyone except after the message has been conveyed. Therefore, the disbelief of ignorance, when the حجت (proof) has not been established and it was not possible to know it, is not subject to punishment. The establishment of the حجت (proof) varies according to times, places, and individuals.

The ruling of apostasy against individuals is not for ordinary people to make, but rather for the legitimate judiciary, which is handled by people of knowledge, after the conditions for declaring apostasy are met and its impediments are absent, such as the person being an adult, sane, acting out of free will, and not excused by ignorance or misinterpretation.