How many cat nip i can find in the game?

One in every game. Get one, start a New Game Plus, get another one, start another New Game Plus, etc. etc. etc.

Catnip?

It's an accessory. You get in a treasure chest for beating the Level 40 Via Infinito boss. It lets either Y, R, or P always hit for 9999 damage on any attacks including multiple 9999 chains, as long as they're in ciritical (yellow) status.

Cat nip Really helped throughout the game. I beat all the bosses with it. I even beat the desert feind by equipping sprint shoes on yuna and I put her in critical status before I entered the battle with the feind.