Your I-94 shows when and where you last entered the United States. Your I-94 number is necessary for various things, including hire paperwork, your driver's license application, your social security number application, your OPT application, and so on. To retrieve your I-94 number, visit the Customs and Border Protection website. You should retrieve your I-94 number as soon as possible upon entering the U.S. You get a new I-94 number every time you reenter. You need to retrieve your I-94 and keep it on file each time you reenter. You cannot retrive your I-94 after 5 years and have to pay a hefty fee to obtain a copy. Therefore, it is important that you print it out and keep it on file whenever you reenter the U.S.
Your I-94 number is 11 digits. If you think you are typing in all the correct information but still cannot retrieve your I-94, see an International Student Advisor.