Elina,
I-94 is the departure record document and normally one would not need the previous I-94 number. It's always a good practice to photocopy the I-94 every time you get a new one.
Just remember to check that I-94 expiration date matches your petition end date(PED on your VISA stamp) in case you are here on a work VISA. If not, you will have to go to the local BCIS office and get the I-94 corrected.
Hope this helps,
W_S_I_D