How many batteries? 1 battery per gun is what i go with. When your ammo is limited to 300-1200 rounds for the full 24 hours a single battery will last you. What is more important is to bring an extra gun that is ready to go and then maybe a 3rd gun. I am not refering to a sidearm either, I mean a back up that is combat ready.
You can never bring too many guns to a fight. You can leave the extra one at the base, or pack it with you if you can.