How to acheive the same volume on all of my commentaries?

Ideally you would use exactly the same studio set-up every time. If necessary, write down which microphone you used, distance from mouth, where you sat, equipment settings … every detail required to get the “studio” set-up the same each time.

Beyond that, it is mostly a matter of practice and experience to be able to hit the same vocal loudness, pitch and intensity on every recording.