Buy as much as u can afford, but based on who u r fighting. If u r fighting attack, buy defence. If u r fighting defence, buy attack, and so on.
It really depends on your build and how you war. I rarely get hit in war bc I battle from pin ... and bc I'm not easy to hit. Bit it never hurts to have extra pots. I never let my pots drop below 500 ea even outside war.