A# is not the number printed on your I-94 card.
If you do not know your A#, you probably do not have one now. You can leave it blank.
A# is an 8 digit number assigned to you by USCIS, in the format of Axx xxx xxx.
You can look at every approval notice that you received from the USCIS. In the "Applicant" or "Beneficiary" box the A# should be printed there if you have one.
Even if you have been assigned an A# for your previous application for OPT, you will be assigned a new A# for your I140/I485 application, and this number will be printed on the I-797 Notice of Action receipt.