If you entered the U.S. before May 7, 2013, you were provided with an I-94 card (Arrival/Departure Record) by U.S.
Customs and Border Protection (CBP) at the port of entry. You can find your I-94 Number on this document.
After May 7, 2013, visitors no longer receive the paper I-94 cards in their passports. However you can go to the U.S. Customs and Border Protection website to access and print out your I-94 information: