99,999 is the max damage you can deal to enemies with any attack, but you will need to do a few things in order to get to that number. When you first start the game, the damage cap is set at 9,999, meaning you will need to unlock the higher damage cap of 99,999. Visit the Break Damage Limit page to find out how to do this. Once you have out of the way, one of the easiest ways to deal max damage is with the Costly Punch Materia. Get your Max HP up high and use this attack to easily hit for 99,999 damage. If you plan to do everything in the game, then this will likely come naturally, since there are plenty of enemies where hitting for that damage is almost required.