All sofa covers are sold by the piece. This price is for 1 sofa cover (not the whole set). In the size options you can choose the right size, usually a set of sofa needs a combination of multiple sizes
The size of the sofa cover you choose should be 20~30cm larger than the actual size of the sofa. The rest can be hung down to protect the sides of the sofa, or tucked under the sofa to prevent slipping.