If you entered the US after May 21, 2013 on any type of non-immigrant visa, you did not get I-94 form attached to your passport.
Starting in May, U.S. Customs and Border Protection (CBP) started implementing the automated version of Form I-94, Arrival/Departure Record, at air and sea ports of entry. If you need to have your I-94 Number, you can get it through this link.
For further information read CBP Begins Implementation of Form I-94 Automation.