One thing no one has mentioned is colorways. You might develop fabrics in certain colors that are just "you." One of my quilting buds is all into jewel tones. I love earth tones. Your stash will begin to grow around your colors and style. BUT you will find odd prints and colors that you just have to have.These will add the WOW factor to your quilts. I do have crayon colors in my stash and they aren't the least bit earthy. I don't have cats but there are dogs, horses and farm animals in my stash. . .oh yes, monkeys too!
Now you are going to bring home lots of yards of something you wonder later why you bought. These are called backing!!
As to amounts, if you find a steal and can afford it and absolutely love it, you buy it all or 6-7 yards. If it's a deal and might work, a yard or two . . .and a half. If it's do die for, price matters little so you buy the largest amount your favorite pattern calls for. In my case 3 1/2 yards. . . at least!
Finally, for me I have to have a yellow gold and a small black and white stripe always at hand. For some reason, these in small doses can empower my otherwise ho hum charity quilts.
Have fun and share your stash building journey with us. We all love to be jealous when you SCORE as you have no doubt noticed!