First time mom. When should I pump ?

I want to start building a stash and have been wanting to. I really need to because I want to start feeding my baby from a bottle at night time for my husband to feed him and when I have to be away from my baby for a few hours.

No I do not just want to pump that day for when I’m gone or anything like that. i WANT to have a stash.

So my baby will eat on say the left side 20 or more minutes, then two hours later eat in the right side for 20 or more minutes.

Should I pump on the side he didn’t eat on after he’s done ? Or when should I do it and which boobs