Why is there no admission number on my I-20?

Pre-Arrival Advising does not enter an admissions number on the I-20. 

After you enter the U.S., you will be able to retrieve the I-94 within 24 hours from the Customs and Border Protection website (I94 - Official Website (dhs.gov)

The I-94 will have an admission number listed on the document.  Each time you enter the U.S. you need to retrieve the most recent I-94.  Each I-94 will have a different admission number. 

Helpful Links: Preparing to Enter the United States - Custom Border Protection | Global Penn State (psu.edu)