I have not seen the most recent I-485, you have to put the B2 Visa number.
I just went through my old I-485, in part 3. Processing information, it asks you the following: in what status did you enter? (Visitor, Student, exchange, alien, crewman. temporary worker, without inspection, etc.) you will put down Visitor, since B1-B2 are issued to visitors.
Then put down the Visa Number, Consulate where it was issued and the date Visa was issued. Let me know if the above was of any help.