Paul had agreed to seek to rectify the slanderous report made against him that he was a renegade Jew, i.e. that he was teaching Jews of the Diaspora to forsake the Law of Moses by not walking according to the manner of the Torah and by ceasing to circumcise their children (Acts 21:21-24). When his seven-day service was nearly complete, Jews from the province of Asia saw Paul in the Temple and stirred up the people against him and seized him with the intent to kill him (Acts 21:27-28, 31).
When a million or more pilgrims visit the Temple during the three major holy day festivals: Passover, Pentecost and Tabernacles, how is it that these Jews from the province of Asia chanced on seeing Paul while he was completing the purification rites with four other Jews? It seems quite odd to me that such a chance event would occur when the crowds in the Temple would be so great. Paul had enemies both in Jerusalem and in other parts of Europe and Asia (Romans 15:31; Acts 19:24-34; 20:3, 19). It was probably inevitable that one of these enemies would hear that Paul was present at Jerusalem during the Pentecost festival (Acts 21:22) and seek him out. So, was it by chance or by design that these particular enemies saw Paul in the Temple?
Luke tells us that these Jews who stirred up the people saw Paul earlier with a gentile, Trophimus, whom they had recognized from the city of Ephesus (Acts 21:28). It would be logical to presume that Paul was being watched by these men and perhaps others whom they informed. If they hated Paul, why would they let the opportunity to stalk him go by, once they saw and recognized him at Jerusalem (Acts 20:19; cp. Act 14:19; 17:13)? An enemy might very well prefer to catch Paul doing wrong and punish him legally, rather than risk lying about a wrong in order to convict Paul of death, because, if the enemy were found lying under oath, then he would receive the death penalty instead of Paul (Deuteronomy 19:16-19). Therefore, it is more likely that the Asian Jews, knowing Paul was in Jerusalem, waited until he had almost completed the purification ceremony (7 days) before accusing him of sacrilege. Not believing in Paul’s integrity, they probably suspected he would try to do just as they accused him. While it is true that they only thought Trophimus was in the Temple with Paul (Acts 21:29), a man with a shaved head would take on a different appearance than normal, so they proceeded with their accusation, no doubt believing Trophimus would be found among those with Paul.
Notice how they worded their accusation. “This is the man who teaches everyone everywhere against our people, our law, and this sanctuary!” (Acts 21:28 – NET – emphasis mine). The accusation presumes many of the people would know to whom they referred. Earlier, James and the elders earlier explained to Paul that the Jews at Jerusalem (including many believing Jews zealous for the Law): “they have been told about you, that you are teaching all the Jews who are among the Gentiles to forsake Moses, telling them not to circumcise their children or to walk according to the customs” (Acts 21:21 – NASB). How the accusation of the Asian Jews is termed makes it appear that they may have been the very people who had been spreading this slanderous report against Paul!
The text presumes knowledge of the Jews from Asia that would normally be impossible to know, i.e. what these Jews had seen earlier, and what they presumed to be true presently in the Temple. Therefore, at some point they must have been publicly interrogated, and it would be within the realm of probability that the four men with shaved heads, who were undergoing rites to release them from their vow, were seized by the authorities for proper identification. When it was certified that Trophimus was not among them who had been with Paul in the Temple, the Jews from Asia made themselves scarce, because their intent to hurt Paul could backfire and hurt them if justice was ever sorted out by the Romans (cp. Acts 24:18-19).
 The number was arrived at by counting the number of sacrifices and allowing no more than 10 people per sacrifice. Josephus numbered the people at a Passover at 2,700,200 (see Wars of the Jews, 6.9.3.