They're referring to the number of immigrants being sponsored by whoever is signing the I-864A. Simply list the immigrants in the table at question 13, and count them. In your case the number for "indicate number" would be 1 since your mother is the only immigrant.
PD: Jan 2003 (EB3 rest of world)
I-485 Approved: July 2007
I am a layman, not a lawyer. What I write here is not official or professional legal advice. In addition, my answers on this forum are specific to the scenarios discussed in each thread and should not be generalized to other situations.