Question: Of the six realms of existence, which has the most sentient beings?
Answer: Numbers in the scriptures were not encountered, probably as they fluctuate with rebirth. However, there is this ratio stated by the Buddha in the Nakhasikhā Sutta:
Human beings : All other sentient beings
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Dirt picked up in a fingernail : Rest of the dirt on the great earth
The sutta further states that the dirt in the nail is ‘next to nothing’. The Buddha taught this to emphasize the great difficulty and thus great rarity to be reborn as humans, which is why we should live heedfully, to train ourselves spiritually.
In the related Paṁsu Suttas, the Buddha taught that there are few beings upon departing from the human realm, who are reborn in it again, with far more beings reborn in the three lower realms of hell-beings, hungry ghosts and animals.
Conversely, in the Dhammapada (Verse 174), the Buddha taught that, ‘Just as birds who have escaped from a net are few, few are the people who make it to the heavens.’
To summarise, most beings fall into the lower realms and few beings reach higher realms, which include that of human beings. This means it is very easy to fall and remain in the three lower realms, and very difficult to rise and stay in the three higher realms. This is so as it is very easy to do little and great evils to karmically deserve fall. Even with some positive karma, it is easy to be depleted due to not living heedfully. Even those in the higher realms of asuras and heavenly beings, if yet to have any form of enlightenment, are likely fall in their next lives.
Thus, the broader answer to the question is that the three lower realms have the most sentient beings. On earth itself, a hint of this can be known, if we just look at the population ratio of human beings versus insects of the animal realm: (Note that this has yet to take other animals into account, what more hungry ghosts and hell-beings.)
7.8 billion humans : 10 quintillion insects
In the current Dharma-Ending Age, due to the greater prevalence of weaker morals while living with stronger temptations, it can be said that even more human beings will fall into the three lower realms. Thus, now, as good as in any other time, it makes great sense to seek birth in Pure Land, to be liberated from falling, and from painful existential oscillations, once and for all.
Related Article:
How Do Beings Ascend From Lower Realms?
https://thedailyenlightenment.com/2019/08/how-do-beings-ascend-from-lower-realms
