I plan to use this step-down voltage regulator (https://www.pololu.com/catalog/product/2110) to power my robot.
I also wanna use 6 AA(A) batteries.
My question is: What is the minimum voltage needed to get a stable 5v output ?
And will 6 batteries be enough, even if they’re (almost) empty ?
You haven’t said the chemistry of your batteries, which is important. However, 6 cells in series should be fine for common chemistries. Inputs above 6V should give very close to a 5V output. When you go lower, the output voltage remains stable, but drops accordingly.