You should contact the manufacturer *not* the seller to find out the maximum current draw for the shield.More about SIMCom SIM908Please contact:Tel: 86-21-32523300Fax: 86-21-32523200Email: email@example.comWebsite: www.sim.com/wm
9v batteries are *terrible* for current output, just awful. Wifi, GPS, and cell communications will typically involve peak current in the hundreds of mA; I wouldn't expect that to work. Use 6 AA batteries in series instead if you need 9v (you could use 4, except that the regulator on the Uno has fairly high dropout, and can't provide 5v off 6v input :-( ), or if you're using it in the car, power it from a car cellphone charger.
Using 9V batteries is a complete waste of time. Even if you put enough in parallel to power the GSM module, they would be very quickly exhausted. Find another power source, or another project.
AA rechargeable, C or D rechargeable or alkaline batteries, automobile 12V with noise reducing adapter, any lead acid battery. AA alkaline batteries will provide 2 A only when fresh.
If this is to be a vehicle tracker and must be battery powered, the battery capacity should be sufficient for double the expected run time.
If the vehicle battery is isolated, you don't need a noise reducing adapter but you may need a voltage regulator. A fully charged lead acid battery can produce over 14V which is outside the range of some Arduino compatible circuitry.If connected to an automotive power source, then while the engine is running there can be spikes of several hundred volts from wiring faults and the ignition circuitry. These can instantly destroy unprotected electronics. Most "cigarette lighter" power adapters have such protection circuitry built in and this would be the easiest way to go. There are lots of DIY automotive noise filter circuits on line.