Three colors of thread is all that is necessary for piecing.
I always have a white/cream, a light tan, and a medium gray. with these three you can piece light, medium, and dark fabrics.
Always buy a good quality thread. I usually use Mettler. But a local quilt shop had Masterpiece by Alex Anderson on sale. So I've been using this thread lately. Cheap thread will leave a lot of lint in your machine. If you are going to do Applique,you will need more thread colors to match your fabrics.