It is impossible to say how many upanishads there are, nor does a Hindu think the less of an upanishad because it is not found in a certain list.
"Hinduism and Buddhism, Vol I. (of 3) An Historical Sketch"
Charles Eliot
The sketch of his doctrines which it contains is clearly a preliminary study elaborated and amplified in the upanishad.
"Hinduism and Buddhism, Vol I. (of 3) An Historical Sketch"
Charles Eliot
The doctrine must therefore have been popularized after the composition of the upanishad.
"Hinduism and Buddhism, Vol I. (of 3) An Historical Sketch"
Charles Eliot