The role of the Buddha in establishing numerous Arahants can’t be overlooked. But even in the Buddha’s day, the downwards trend of Arahants was noticed.
SN 16.3On one occasion the Blessed One was staying near Savatthi in Jeta's Grove, Anathapindika's monastery. Then Ven. Maha Kassapa went to the Blessed One and on arrival, having bowed down to him, sat to one side. As he was sitting there he said to the Blessed One, "What is the cause, lord, what is the reason, why before there were fewer training rules and yet more monks established in final gnosis, whereas now there are more training rules and yet fewer monks established in final gnosis?"
"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.[1]
But Buddhaghosa had a formula for calculating the number of people who reached absorption in jhana.
That implies a lower limit of 1 in 1 million and an upper limit of 1 in a billion.[The] preliminary work is difficult for a beginner and only one in a hundred or a thousand can do it. The arousing of the nimitta is difficult for one who has done the preliminary work and only one in a hundred or a thousand can do it. To extend the nimitta when it has arisen and to reach absorption is difficult and only one in a hundred or a thousand can do it."2
Given the upper limit, there may as many as 8 or 9 people living today who are able to reach absorption.
In 500 BC the population of India was 35 million. Given the lower limit, 35 people may have been able to enter jhana at the time the Buddha taught.
Given any upper limit or even a middle approximation, zero people in the Buddha’s time could have reached jhana.
And of course in 500 AD (when Buddhaghosa lived) 250 people could have reached jhana.
The paradox of course is that the Buddha already explained that as time goes by and the true dhamma disappears less and less people reach jhana. But according to Buddhaghosa, as time goes on, the population grows, and thus so too does the probability of someone reaching Jhanas.
The Mathematics of Buddhaghosa are wonky to say the least. At best, they are at odds with the teachings of the Buddha.