I think seven.
Alicorns (between 4 to 7. See number of Alicorns)
In "The Elements of Harmony" encyclopedic book on pg. 53 it describes Changelings as follows:
"Changelings are grotesque shape-shifting ponies who can morph their bodies into anything. These frightening creatures have fangs as well as insect features and translucent wings designed to scare even the bravest pony into submission."