The old dope pedlar and his powdered happiness.

How many angels can dance on the head of a pin is indeterminate for two reasons:
As magical beings they can be any size they choose; and,
As musicians, they provide the music for the dancers. The second suggests that they don't dance but may have the capability when not performing . . . should they choose to do so.
Unless they are endowed by their creator with a sense of rhythm upon translation, there many who would be poor musicians or dancers.