I bought a big bag of yo-yos last summer at a yardsale...and i asked myself the same question! There is a paper in the bag with the number 780. That is probably how many are in there. There is also a stack of circles cut that is 3" high. The finished yo-yos are about 2" across.
A friend brought a yo-yo quilt to our quilt group last month for show and tell. It may have just been what i needed to decide to put one together myself. I had considered selling them, but i liked that quilt she showed.