Please answer a question about the following article about Buddhism:

Buddhists do not consider Siddhartha Gautama to have been the only Buddha. The Pali Canon refers to many previous ones (see List of the 28 Buddhas), while the Mahayana tradition additionally has many Buddhas of celestial, rather than historical, origin (see Amitabha or Vairocana as examples, for lists of many thousands Buddha names see Taishō Shinshū Daizōkyō numbers 439–448). A common Theravada and Mahayana Buddhist belief is that the next Buddha will be one named Maitreya (Pali: Metteyya).

How many Buddhas are considered to have existed in the Pali Canon?
28