Each year, the Department of Homeland Security (DHS) generates a "report to provide data on departures and overstays, by country, for foreign visitors to the United States who were expected to depart in each Fiscal Year."
DHS posts the annual overstay reports on its Entry/Exit Overstay Report page.
The most recent Entry/Exit Overstay Report is dated September 30, 2021, and reports on fiscal year 2020 data. The 2020 Entry/Exit Overstay Report cites the COVID-19 pandemic as a reason for an increase in the number of "overstays" calculated in 2020 compared to 2019.
8 USC 1376 requires DHS to: "implement a program to collect data, for each fiscal year, regarding the total number of aliens within each of the classes of nonimmigrant aliens described in section 1101(a)(15) of this title whose authorized period of stay in the United States terminated during the previous fiscal year, but who remained in the United States notwithstanding such termination" and to each year (no later than June 30), "submit an annual report to the Congress providing numerical estimates, for each country for the preceding fiscal year, of the number of aliens from the country who are described" above.
8 USC 1365a(e) requires DHS to use its "integrated entry and exit data system... to prepare an annual report to the Committees on the Judiciary of the House of Representatives and of the Senate" that includes "the following information with respect to the preceding fiscal year, and an analysis of that information:
(A) The number of aliens for whom departure data was collected during the reporting period, with an accounting by country of nationality of the departing alien.
(B) The number of departing aliens whose departure data was successfully matched to the alien's arrival data, with an accounting by the alien's country of nationality and by the alien's classification as an immigrant or nonimmigrant.
(C) The number of aliens who arrived pursuant to a nonimmigrant visa, or as a visitor under the visa waiver program under section 1187 of this title, for whom no matching departure data have been obtained through the system or through other means as of the end of the alien's authorized period of stay, with an accounting by the alien's country of nationality and date of arrival in the United States.
(D) The number of lawfully admitted nonimmigrants identified as having remained in the United States beyond the period authorized by the Attorney General, with an accounting by the alien's country of nationality."
Both these provisions arise out of the Illegal Immigration Reform and Immigrant Responsibility Act of 1996 (IIRIRA) and its subsequent amendments.