References – SN 12.4: Vipassī 
“Please explain what gave rise to the 28 Buddhas Protection Discourse (Amaravati Chanting Book Volume 2, p. 54). Is this a sutta? It didn’t start with ‘Thus have I heard.’ Did Buddha ever mention or acknowledge the existence of other Buddhas?” Answered by Ajahn Pasanno, Ajahn Karuṇadhammo and Ajahn Yatiko. [Protective chants] [Sutta] [Previous Buddhas] // [Metteyya Bodhisatva] [Culture/Thailand] [Ghost] [History/Sri Lankan Buddhism] [Children] [Fear]
Sutta: DN 14: Descriptions of six previous Buddhas.
Sutta: DN 26.25: The future Buddha Metteyya.
Sutta: DN 32: The Āṭānāṭiya Protection.
Suttas: SN 12:4-10 mention six previous Buddhas who understood Dependent Origination.