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.
There is a word that begins with "Z" and means love/loving. That word is Zoriothidist!
zanderfide
Zealot
zeal, zealous
zany
Zenith.
zatanous
A word relating to animals is Zoology.
zeal
zoology
Zealous is a word that means the same as happy and starts with a Z.