Really depends what you want your quilt to be.
Some just want to have one of each thing ... Eg. I spy a cat! And there is only one cat to be found.
Whereas some want their quilt to help their little one learn to count. Eg. I spy three cats! The quilt may just have those three cats. And maybe even more, so that another time the child can be asked to find five. Or Whatever!
Do what works for you, and the recipient!
Aside from what "I Spy" fabric choices you make, there's so many different designs/layouts for I Spy's! Look around here on the QB and do some googles. And you'll have so many ideas floating in your mind.
Have FUN!!