You gain around 10 RPG ammo per full playthrough of the game. Chapter 3 is good for this on New Game +. There are plenty of enemies in the second area with the chainsaw sadist. Sebastian has to kneel and can't move when using the weapon, so make sure you're out of the haunted's reach - but not too far away because the rockets are a bit unreliable. Reloading autosave will keep your kills, but will not replenish ammo.