In the NES version, items are replenished after you leave an area, so you can take the same slice of pizza over and over. This is not necessarily true of other versions of the game.
Each weapon upon being picked up will supply 20 of that weapon.
Shuriken
Triple Shuriken
Does 1 bar of damage. Travels all the way across the screen.
Triple shuriken - Same as shuriken but you get a three-shuriken spread. Since they're better than standard shuriken, they're less common.
Boomerang
Kiai
Does 1 bar of damage. They only travel a short distance before going in reverse. If you catch it, you regain it. If you don't, it travels across the screen exactly as a shuriken. If you switch characters and catch the boomerang, that switched character will obtain the boomerang, replacing other inventory.
Does a whopping 2 damage. These scrolls only appear in certain locations and cannot be retrieved by enemies, unlike the other 3 weapons. They travel across the screen and covers a wide range. The best weapon in the game.