If you wanted yellow, id say at least 3. But for any red based (red, org or purp), 1 is usually plenty.
For green, lemon/lime, for orange, orange and for purple, grape. If you can find a gn apple (i think thats the flavor), thats more a mint green, but its d/c.
Buy 1 more than you think you need. They last forever if you dont intend to drink them. Its easy enough to remove the silk, add more dye, dissolve and put the silk back in if you want it darker. But having to rinse out a ton of excess dye is a pain, and if its too dark, not a lot you can do about it.
It depends on how big the silk is too. For a small 20x20, I might only use one packet but for a larger 35x35, I'd use two. And depends how dark you want it too. I've used grape to get purple, but I've also mixed one cherry and one berry blue and gotten almost the exact same purple color.