Hi Fernando! I think it depends on the types of quilts you like to make. Personally, I like to make quilts with LOTS of different fabrics, and I tend to purchase fat quarters so I have a large selection. I also make a lot of applique quilts and sometimes I just need a tiny amount of some colors. If I see something that I really like, I may get a yard or two (for sashing or border strips or backgrounds) and ocassionally if I see something I really LOVE I'll get 6 yards to use for a backing and binding. (I quilt on a long-arm machine and need length of backing to mount it on the frame.) If I'm only attracted the colors, I'll just get a fat quarter to use it for a focus fabric and pull fabrics from my stash that will give me the flavor of that fabric for the overall look of my quilt.
Be aware too, that your tastes will probably change over time, so don't fill your cabinet with similiar fabrics!
I also suggest that you try to store your fabric so that you can tell at a glance when you are getting low on a color. Need more greens? Some orange?
Probably the most important thing to pay attention to is the value of the fabric. I don't mean money, but the lightness or darkness of the colors. You need a good mix of light, medium and dark fabrics available in your stash.
Have fun! And remember to save room for new stuff! The most gorgeous fabric you've ever seen may not even be printed yet!!