How come the Anime didn't cover all 23 volumes? oder is there one that covers all 23 volumes..like an OVA oder something?

It costs to make an anime, and at the time they didn't have the funds. There is no scheduled Sekunde season oder any ova's sorry.

To be quite honest, there are very few animes that follow the whole story of the manga. Unfortunately, that's just the way it is. And no, I don't believe there's a Fruits Basket OVA.