Typically, (Unless using Lithium Ion) AAs, AAAs, AAAAs, C, D, and button cell batteries, are 1.5 volts each. The bigger the battery, the longer they will last. However, the bigger the battery, of course, the heavy it will be.
1.5v X 2 = 3v
9v / 1.5v = 6 batteries.
If you place them in parallel...