Read the following paragraph and answer the question. Give ONLY the answer, no other words or explanation.

For example:

Answer: <most likely answer, as short as possible>.

Paragraph: 
Of course. But what type? Of the nine classes of vertebrates, five are fish. Each of the five classes has distinguishing characteristics that allow members to be classified appropriately. Stingray are cartilaginous fish, related to sharks.


Question: Of the nine classes of vertebrates, how many are fish?
five
5.