How much weight drywall can support varies on several factors. First is thickness. Drywall comes in three standard sizes; 5/8, 3/8, and 1/4. Obviously, thicker is sturdier. Drywall is surprisingly sturdy and can support up to roughly 40 pounds on a single anchor stud, but the key to strength is to spread the weight out over multiple anchors, the objects you drill into the wall to hang things on that come in a variety of sizes and strengths. Using more of them distributes the weight out and lessens the burden on the individual one. So, heavier objects, like mirrors, will need more anchors. Different types of anchors include: toggle bolt, threaded toggle, molly bolt, threaded anchor, and the various types of picture hangers. Hanging objects from a wall stud (the wood borders that make up your wall and sit behind your drywall) will provide maximum strength.