How much you need depends on your rode and the conditions.
It always amazed me that if we did everything right, an anchor that weighs something on the order of one-half of one percent of what our boat weighs could hold us in place through 50-knot winds. That it can do so depends almost exclusively on the angle of pull on the anchor. Pull any anchor straight up and, unless it’s jammed on a rock, it doesn’t take much to get it off the bottom.
The amount of rode you put out, or scope, combined with weight at the anchor end of the rode, determines the angle of pull on the anchor. More rode is always better, as is more weight. Paying out more rode decreases the angle of pull against the anchor, making the anchor dig itself in the more the boat pulls against it — a good thing. The same benefit will occur when the wind shifts. Too little scope, and the movement of the boat in the shifting wind can jerk your anchor out. More scope is almost always your friend. How much rode you absolutely must have depends on the conditions and on how much weight you have in the rode. The illustrations above show the minimum amount of scope for different conditions with different types of rode.