Basically the number on your ID/DL or your SSN.
A "government issued id number" is any number issued by the government which is intended to identify an individual.
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Self filed AOS │Apps Received By USCIS - 3/18/19
│Biometrics Done - 4/11/19
Interview Scheduled - 4/24/19│Interview Date - 5/31/19│AOS Approval - 5/31/19
Permanent Resident Card Received - 6/8/19