Relatively speaking, there are countless sentient beings.
Ultimately, however, nothing, not one thing can be found that has intrinsic "being-ness".
Thus, it's the realization of no-self that is experienced as liberation.
Chapter 25 of The Diamond Sutra:
“Subhuti, do not say that the Buddha has the idea, ‘I will lead all sentient beings to Nirvana.’
Do not think that way, Subhuti. Why?
In truth there is not one single being for the Buddha to lead to Enlightenment.
If the Buddha were to think there was, he would be caught in the idea of a self, a person, a living being, or a universal self.
Subhuti, what the Buddha calls a self essentially has no self in the way that ordinary persons think there is a self.
Subhuti, the Buddha does not regard anyone as an ordinary person.
That is why he can speak of them as ordinary persons."
Conventionally, we can refer to individuals, various different beings,
because that is the appearance of things which arises as a projection of mind in ignorance.
It's like, different streams and rivers may all lead to the sea. But the water is water is water.
Fundamentally, there is no difference (even if some of it is salty)
and the sea is also the clouds and the rain from the clouds is also a watermelon.
It's only due to where we are observing things at any given time,
that the water appears to be different, that a watermelon appears different than a rain cloud.
The ocean overcomes (transcends) whatever individual differences appear between this river and that river or stream.
Likewise, we perceive different beings, different realms, and so on,
but those differences are not intrinsic or permanent, but rather, merely conditional and temporary.
One Buddha is the same as a billion buddhas.