I would like to tell you there are zillions of words that mean "much" and start with the letter Z. I would like to tell you that, and then I would like to list them, and this for both our sakes. But the word for which I think you seek is the one I put in the first sentence of my reply. "Zillions" will probably do you. P.S. When they ask you for a synonym for zillions that begins with the letter "O"--try "oodles."
zenith is a complementary word that starts with z. It means on the top, or the best.
One word that starts with "z" and means love is "zeal." It signifies enthusiasm, passion, and intense ardor towards something or someone.
zanderfide
Zealot
Zenith.
zatanous
zeal, zealous
zany
A word relating to animals is Zoology.
zeal
zoology
Zealous is a word that means the same as happy and starts with a Z.