You can blend shapes with the same or different number of vertices or CVs.

When you create a blend shape deformer, you should turn the Check Topology creation option off if you want to blend objects that have different numbers of CVs or vertices. For more information on Check Topology, see Create Deformers > Blend Shape.

If objects have the same number of CVs or vertices but their order is different, Maya blends the shapes whether Check Topology is on or off. However, the position of the base CVs will be transformed to the position of the target CVs. This change might cause the object to blend in a way you might not expect. To ensure a smooth transition between base and target, make sure the order of CVs is the same in both objects.

In addition to blending individual objects, you can blend hierarchies of objects. Make sure both hierarchies have the same number of children and parenting relationships.

To blend hierarchies, you must select the parent of the target hierarchy (or hierarchies) first and the parent of the base hierarchy last before creating the blend shape. The parent of each must be a transform.

Each child in the base blends into its corresponding child in the target. The order of children in the Outliner (and Hypergraph) determines which children blend. If necessary, use the Outliner to change the order of objects in the hierarchies.

A common blend shape technique is to create duplicates of a base, deform the duplicates, then use them as targets. For example, you might make several copies of a face, and then alter the copies to create a smiling face, frowning face, a crying face, and so on. If you use this technique, turn on the Check Topology creation option when you create the blend shape deformer. This checks that the base and target hierarchy shapes have the same number of CVs. If the CVs are different and Check Topology is off, you might see, for instance, an eye blending into the nose. If Check Topology is on, the members of the hierarchies must have corresponding numbers of CVs.