typedef enum max31865_numwires {
MAX31865_2WIRE = 0,
MAX31865_3WIRE = 1,
MAX31865_4WIRE = 0
} max31865_numwires_t;
This defines a type max31865_numwires_t. The _t at the end is a clue to tell you that it's a type. (The compiler doesn't care.)
So just the same as int i; creates an integer called i, max31865_numwires_t z creates a numwires called z.
You don't put max. in front of it as this is defined outside of the class.