Answer:
It is unknown. Only thew IRS can develop an estimate that is at all reliable, and as yet, it has not done so. Estimates publshed to date are derived from sources not in a position to judge, and are repeated by those who like what they hear, and ignored by those who do not. Speaking as someong with some knowledge of internal IRS operations, the often quoted 16,000 figure sounds far to high by a wide margin.