This answer is for a practical application.
Connecting batteries in parallel is never a good idea. They simply cannot be made to have *exactly* the same internal resistances and so one battery will always supply more current than the other.
Since you run that robot for such a short time why not just use 4 x 1.5V 'C' cells in series? You get a little above 6V when they are new, using alkaline cells, you should get enough current until the voltage drops too low for your servo motor. Or use 5 NiCad C cells in series and run that machine for much longer.
Now, if you are making a theoretical question (such as an assignment) and you *must* use an impractical number of batteries, choose the 8 x C-cell ones.
There is not enough grunt in the little 9V batteries.
Use a voltage regulator that can handle at least 1 Amp of current. You could use a variable type or a fixed 5V one, the latter padded up with 2 diodes to output 6V.
Klaus