There is some information here at wikipedia
I am curious to know if that came from Buddha himself or mentioned in any suttas. The 'sasana' concepts restricts appearence of solitary buddhas or Prateyakabuddhas or paccekabuddhas (as described in Isigili Sutta and Rhinocerous Sutta) without a Sangha. Any thoughts...
Here are some pali sutta references.
Ānanda, if women had not obtained the Going-forth from the home life into homelessness in the Dhamma & Vinaya made known by the Tathāgata, the holy life would have lasted long, the true Dhamma would have lasted 1,000 years. But now that they have obtained the Going-forth from the home life into homelessness in the Dhamma & Vinaya made known by the Tathāgata, the holy life will not last long, the true Dhamma will last only 500 years. https://www.dhammatalks.org/suttas/AN/AN8_51.html
“That’s the way it is, Kassapa. When beings are degenerating and the true Dhamma is disappearing, there are more training rules and yet fewer monks established in final gnosis. There is no disappearance of the true Dhamma as long as a counterfeit of the true Dhamma has not arisen in the world, but there is the disappearance of the true Dhamma when a counterfeit of the true Dhamma has arisen in the world. Just as there is no disappearance of gold as long as a counterfeit of gold has not arisen in the world, but there is the disappearance of gold when a counterfeit of gold has arisen in the world, in the same way there is no disappearance of the true Dhamma as long as a counterfeit of the true Dhamma has not arisen in the world, but there is the disappearance of the true Dhamma when a counterfeit of the true Dhamma has arisen in the world. https://www.dhammatalks.org/suttas/SN/SN16_13.html#sn16.13note01
I think it's natural to assume that 500 years refers to the appearance of counterfeit Dhamma but there are people who believe otherwise.
There is a lot of controversy around how disappearance comes about.