Depends on how long and hard you want to run it.

A Pi5 uses a 5V 3A charger. Four AA batteries in series should give you 4-6V (the voltage changes as they discharge) and around 2A (going by the labelling on my rechargeables). So you might be able to run a Pi5 at full throttle for an hour or so given 8 AAs. There would be some loss at the buck-boost converter you'd need to guarantee an output of exactly 5V.

Using four Ds instead would simplify the wiring a bit and keep it running longer.