how do i figure that out?
Start looking at the items your going to run. They will all have spec sheets telling you the min and max voltages they use and how much current they ca draw. Find the max current for all items and add them up. Now you know how much current they will all pull if they are all running maxed out and doing so at the same time.
If you have everything running full bore constantly then yes a 9V can be drained quite fast. But a 9V is a good place to start. If run time becomes a concern you can always turn to using 4 to 6 D cell batteries. Though a better option may be using a 9.6V battery pack from an RC car. Those are made for running heavy loads like motors running hard.