in this aeon alone, there have been 4 sammāsambuddhas, including gotama.
‘It is now ninety-one aeons ago, brethren, since Vipassī, the Exalted One, Arahant, Buddha Supreme, arose in the world. It is now thirty-one aeons ago, brethren, since Sikhī, the Exalted One, Arahant, Buddha Supreme, arose in the world. It was in that same thirty-first aeon, brethren, that Vessabhū, the Exalted One, Arahant, Buddha Supreme, arose in the world. It was in this present auspicious aeon, brethren, that Kakusandha, the Exalted One, Arahant, Buddha Supreme, arose in the world. It was in this auspicious aeon, brethren, that Koṇāgamana, the Exalted One, Arahant, Buddha Supreme, arose in the world. It was in this auspicious aeon, brethren, that Kassapa, the Exalted One, Arahant, Buddha Supreme, arose in the world. It is in this auspicious aeon, brethren, that now I, an Arahant, Buddha Supreme, have arisen in the world.’
https://suttacentral.net/en/dn14
as you see, this is a dramatic difference. not only is it uncommon to have a ssbuddha in your present aeon, but it is exceedingly rare to have 4. metteyya is the upcoming ssbuddha, and from what i understand he is coming in this aeon, albeit many, many years from now.
http://what-buddha-said.net/library/Met ... imet04.htm
https://suttacentral.net/en/dn26