All sentient beings, and all Tathāgatas, [are] together all [of] one Dharma, [of the] equal Dharma body [i.e. Dharmakāya], further, without [an]other body, likewise without [an]other mind, [in] principle equal, just as empty space, always abides [as] one ‘form’ [without appearance], without having differences.
– Śākyamuni Buddha
(Sūtra [Of The] Dhāraṇi [Of The] Treasure Siddhi [For] Accomplishing Buddhahood)
In the ‘Sūtra [In Which The] Buddha Speaks [Of] Neither Increase Nor Decrease’《佛说不增不减经》, Śāriputra (舍利弗) asked Śākyamuni Buddha (释迦牟尼佛) if sentient beings (有情众生), who have been reborn beginninglessly, increase or decrease [in number]. The Buddha replied that to see the realm of sentient beings (sattva-dhātu: 众生界) to increase or decrease is a severely wrong (or evil) view (邪见), that arises from not seeing the one Dharma Realm (i.e. 一法界: the whole universe) in accordance with reality, due to not realising the emptiness of all dharmas (i.e. 诸法空性: empty nature of all phenomena, including that of sentient beings). [Sentient beings are after all but phenomena of mind and matter, both of which are impermanent (无常) due to constant change, thus without a fixed self (无我) within these aggregates’ changes.]
When foolish sentient beings hear about a Buddha-Tathāgata (佛如来) entering Parinirvāṇa (i.e. 入涅槃: relinquish physical form after completing his teaching), they cling to the wrong view of cessation (断见) or extinction (灭见) of the Buddha, mistaking the realm of sentient beings to decrease accordingly (as if it now has one ‘less’ sentient being, when all that ceases or becomes extinct are the Three Poisons of greed, hatred and ignorance [贪瞋痴三毒], which are the causes of all afflictions [烦恼], and to be trapped in the cycle of birth and death).
From the wrong views of decrease and increase, many more related wrong views follow. [These are roots of defilements (惑: afflictions) that arise from ignorance (无明), as wrong ways of viewing sentient beings, including oneself and others.] Such wrong views can lead sentient beings to have no motivation to progress in Dharma learning and practice – even if seven Buddhas are to successively appear to expound the Dharma to them.
The realm of sentient beings is the Tathāgata Treasury (Tathāgata-Garbha: 如来藏), which is the Dharmakāya (Universal Body of all Buddhas with Buddha-Nature [佛性] realised, and all sentient beings with Buddha-Nature unrealised), not separate or different from all Tathāgatas’ meritorious virtues (功德) and wisdom (智慧). This is just as a lamp is not separate from its light, and a gem is not separate from its lustre.
Since the Dharmakāya is with neither birth [or increase] nor death [or decrease], and is permanent without change, as the [ultimate], cooling (清凉) and equal (平等) refuge (皈依) [for all Buddhas and sentient beings], it is free from dualistic differentiation (不二/无分别).
When the Dharmakāya is [not yet realised, thus] fettered by beginningless afflictions in the cycle of birth and death, one is called a sentient being. When tired of such suffering, and practising the Dharma to attain Buddhahood, one is called a Bodhisattva. When unfettered, one realises the pure Dharma-Nature (Dharmatā: 法性) to become a Tathāgata.
Thus, the realm of sentient beings is not apart from the Dharmakāya, with these three aspects (of the Dharma) of True Suchness (Bhūta-Tathātā: 真如) without differences:
 The Tathāgata Treasury’s original nature corresponds (sambaddha) with the essence of the pure Dharma. It is not separate from reality, wisdom, purity and True Suchness, and is beginningless. [The Tathāgata Treasury is not empty as it is self-nature’s pure mind (prakṛti-pariśuddha-citta: 自性清净心).]
 The Tathāgata Treasury’s original nature does not correspond (asambaddha) with the essence of afflictions tainted (āgantuka kleśa: adventitious or ‘visiting’ afflictions [客尘烦恼]) with impure dharmas. They can only be severed by Tathāgata’s Bodhi wisdom [from awakening as Buddhas]. [The afflictions are empty as they are not self-nature’s pure mind.]
 The Tathāgata Treasury’s existence has the nature of being equal for all, is permanent, cooling and changeless refuge, with neither birth nor death. It is the root of all dharmas, that gathers all dharmas, nor separate from reality, [while afflictions neither arise from it nor defile it].
Relying upon these three aspects of the Dharma, we will never fall to the two extreme wrong views, as we will see all according with reality. Therefore, sentient beings do not increase or decrease, as they are empty in nature, with no fixed beings in the first place, only with continual change in mind and matter in the past and future, as long as unawakened. This is while the true essence of sentient beings is their Buddha-Nature (or Dharma-Nature, or Tathāgata Treasury, or Dharmakāya), which is permanent, and does not increase or decrease, and is essentially universal as ‘one’, without any attachment to self.
A related teaching is also found in the Heart Sūtra《心经》– ‘All dharmas are characteristics of emptiness: not arising (arisen), not ceasing (ceased); not defiling (defiled), not purifying (purified; pure); not increasing (increased), not decreasing (decreased).’ (是诸法空相，不生不灭， 不垢不净，不增不减。) All dharmas include sentient beings too. While all beings neither increase nor decrease, they are not static. In fact, they neither increase to be fixed beings, nor decrease to be no beings, precisely because they are dynamic all the time. What remains unchanging is their Buddha-Nature.
A related question is, ‘How many sentient beings are there?’ There are many ways to answer this – [i] There are no beings as there are no fixed beings. [ii] There is only one (key) being, who is you, who cling to the wrong view that there are many beings, including you. [iii] There are many other beings, in relation to you, as long as you cling to the one (key) being. [iv] There are immeasurable beings, due to you and many others changing swiftly from moment to moment. (‘Sentient beings’ [众生] abbreviates ‘from many causes and conditions as arose and as ceased’ [众因缘所生所灭].)
Thus, sentient beings ‘re-arise’ conditionally; and not permanently, or their Buddhahood would be impossible, with them always being sentient beings. When they become Buddhas, Buddhas ‘arise’ yet do not arise too, only realising their ever-present Buddha-Nature. Thus are Buddhas called Tathāgatas (如来), who are ‘As If Come [And Gone Ones]’, and ‘As If Not Come [And Gone Ones]’.
Sentient beings are likened to gold ore, found naturally mixed with defilements, that can be refined to be pure gold. This analogy based on nature to express potential for perfection makes more sense than to imagine all being already ‘perfect’ in the ‘first’ place, yet illogically still fallible, or to imagine all being totally ‘imperfect’ in the ‘first’ place, yet illogically still ‘perfectable’.
Perhaps, the opening question arose due to thinking that if sentient beings can decrease, all beings can be delivered, with no more to be done after. However, this seems like an issue only if we remain as sentient beings, and will become a non-issue with advancement towards Buddhahood. As the Buddhas have realised the empty nature of all sentient beings, it does not matter to them, even if there are ‘countless’ beings to be delivered, because the natural functioning of realised Buddha-Nature is to keep delivering beings with perfect skilful means of compassion and wisdom, yet always with great ease and bliss. This is why Buddhahood is such an important and worthy goal.
True Suchness [does] not decrease;
True Suchness [does] not increase,
[the] Dharma Realm [does] not decrease;
[the] Dharma Realm [does] not increase,
All Sentient Beings’ Realm, [does] not decrease, [and does] not increase…
– Śākyamuni Buddha
(Sūtra [In Which The] Buddha Speaks [Of] Mañjuśrī’s Patrol)
Buddha Pronounces the Sūtra of Neither Increase Nor Decrease
How Not To Despair At Many Beings To Save?