Two 9 x 12 drop cloths. That's all it took to update an old sofa. The 12oz canvas is quite heavy, but has a nice smooth weave and very few imperfections. A "formal" drop cloth, if you will.
My customer bought them at Sherwin Williams but also found them online Here. $22 each. For real. She pre-washed and ironed them, so I can't say how badly they wrinkled after washing, but at that price, it's worth a little extra effort.
*Update (4/10/15): I ordered two 9x12 12oz. dropcloths from thepaintstore.com. Theyarrived today and aren't the same as the material used for this sofa. They are heavy and nice, but the texture is not as smooth. I'll update again after they've been washed.