If you take time and apply effort to actually penetrate and understand Buddha's teaching, you will see that Nirvana is a metaphor for the perfect realization of the Noble Path, rather than a lottery that one has a certain odds of winning.
The Noble Path is a training system that gets one away from suffering and towards peace, and here's an important detail: it is a gradual system - first you stop the worst causes of suffering then the more tricky and subtle ones and so on. So, like, first we stop physical and verbal violence, lying, sowing conflicts, getting intoxicated, dealing in weapons and drugs, stealing, and other such obvious sources of dukkha. Then we learn to overcome negative thinking, obsessive desires, anger, hatred towards the world, jealousy, and other inner sources of dukkha and learn to be in a nice (positive, confident) state of mind. Then we learn to see and transcend our own prejudices, stereotypes, generalizations, reifications, assumptions, expectations, and other conceptual sources of very subtle very fundamental dukkha. At the end of this process we learn to transcend even the dichotomy of dukkha/sukha itself. What's left at this point is a very refined, very mature, very wise, very robust, stable, and peaceful type of intelligence that does not have a single point of reference. This is what we lovingly and somewhat humorously call attainment of Nirvana. It's not a lottery, it's a culmination of behavioral, emotional, and mental development.
So in terms of how many people can reach it, I don't think it's one in a million or anything like that. It is about all the people who understand the path, follow the path, and methodically refine one's practice of the path.
Just like with the hygiene: it's not about how many people can be 100% perfectly germ-free, it's about the overall principle that cleanliness and sanitization reduces the spreading of pathogens.
Same with Buddhism, whether an individual person gets 100% perfect dukkha-free or only 95% or even 60% is not important in the grand scheme of things, what's important is the shared understanding that realization of the Path is fundamentally beneficial to humanity in general and to each of us, because it reduces suffering and the causes of suffering for all.