In the Bhagavad-gītā (8.17-18) it is stated that according to human calculations one day of Brahmā is equal to one thousand ages of four millenniums (4,300,000 years) each, and the same period is calculated to be his night also. A Brahmā lives for one hundred such years and then dies. A Brahmā, who is generally a great devotee of the Lord, attains liberation after such a downfall. The universe (called the brahmāṇḍa, or the round football-like domain controlled by a Brahmā) is thus annihilated, and thus the inhabitants of a particular planet, or of the whole universe, are also annihilated. Avyakta, mentioned here in this verse, means the night of Brahmā, when partial annihilation takes place and the living entities of that particular brahmāṇḍa, up to the planets of Brahmaloka, along with the big oceans, etc., all repose in the belly of the virāṭ-puruṣa. At the end of a Brahmā's night, the creation again takes place, and the living entities, reserved within the belly of the Lord, are let loose to play their respective parts as if being awakened from a deep slumber. Since the living entities are never destroyed, the annihilation of the material world does not annihilate the existence of the living entities, but until liberation is attained one has to accept one material body after another, again and again.
(Srimad Bhagavatam------2:6:11------purport).

There are so many universes, so many universes. And it is confirmed in the Brahma-saṁhitā: yasya prabhā prabhavato jagad-aṇḍa-koṭi (Bs. 5.40). Jagad-aṇḍa. Jagad-aṇḍa means this universe. Brahmāṇḍa, jagad-aṇḍa. Koṭi, innumerable. Jagad-aṇḍa-koṭi. Yasya prabhā prabhavato jagad-aṇḍa-koṭi-koṭiṣv aśeṣa-vasudhādi-vibhūti-bhinnam (Bs. 5.40). And each universe there are innumerable planets. And each planet is different from others. This is God's creation. And these rascals are manufacturing God. And there is other description also. Yasyaika-niśvasita-kālam athāvalambya jīvanti loma-vilajā jagad-aṇḍa-nāthāḥ (Bs. 5.48). This jagad-aṇḍa, the universe. And there, in each universe, there is a controller of the universe, Brahmā. He is called jagad-aṇḍa-nātha, master of this universe. Because it is controlled by Brahmā, therefore each universe is calledbrahmāṇḍa. It is controlled by Brahmā. So in the Brahma-saṁhitā we get information: jagad-aṇḍa-nāthāḥ, plural number. There are many, many thousands jagad-aṇḍa... Not only one Brahmā. Many millions of Brahmās, many millions of Śiva, many millions of Viṣṇu, many millions of sun. Because there are many millions of universes.
(Lecture on Bhagavad-Gita------1:24-25------purport).

“"Here (in Śrīmad-Bhāgavatam) ten subjects are described: (1) the creation of the ingredients of the cosmos, (2) the creations of Brahmā, (3) the maintenance of the creation, (4) special favor given to the faithful, (5) impetuses for activity, (6) prescribed duties for law-abiding men, (7) a description of the incarnations of the Lord, (8) the winding up of the creation, (9) liberation from gross and subtle material existence, and (10) the ultimate shelter, the Supreme Personality of Godhead. The tenth item is the shelter of all the others. To distinguish this ultimate shelter from the other nine subjects, the mahājanas have described these nine, directly or indirectly, through prayers or direct explanations."
These verses from Śrīmad-Bhāgavatam (2.10.1–2) list the ten subject matters dealt with in the text of the Bhāgavatam. Of these, the tenth is the substance, and the other nine are categories derived from the substance. These ten subjects are listed as follows:
(1) Sarga: the first creation by Viṣṇu, the bringing forth of the five gross material elements, the five objects of sense perception, the ten senses, the mind, the intelligence, the false ego and the total material energy, or universal form.
(2) Visarga: the secondary creation, or the work of Brahmā in producing the moving and unmoving bodies in the universe (brahmāṇḍa).
(3) Sthāna: the maintenance of the universe by the Personality of Godhead, Viṣṇu. Viṣṇu's function is more important and His glory greater than Brahmā’s and Lord Śiva's, for although Brahmā is the creator and Lord Śiva the destroyer, Viṣṇu is the maintainer.
(4) Poṣaṇa: special care and protection for devotees by the Lord. As a king maintains his kingdom and subjects but nevertheless gives special attention to the members of his family, so the Personality of Godhead gives special care to His devotees who are souls completely surrendered to Him.
(5) Ūti: the urge for creation, or initiative power, that is the cause of all inventions, according to the necessities of time, space and objects.
(6) Manv-antara: the periods controlled by the Manus, who teach regulative principles for living beings who desire to achieve perfection in human life. The rules of Manu, as described in the Manu-saṁhitā, guide the way to such perfection.
(7) Īśānukathā: scriptural information regarding the Personality of Godhead, His incarnations on earth and the activities of His devotees. Scriptures dealing with these subjects are essential for progressive human life.
(8) Nirodha: the winding up of all energies employed in creation. Such potencies are emanations from the Personality of Godhead who eternally lies in the Kāraṇa Ocean. The cosmic creations, manifested with His breath, are again dissolved in due course.
(9) Mukti: liberation of the conditioned souls encaged by the gross and subtle coverings of body and mind. When freed from all material affection, the soul, giving up the gross and subtle material bodies, can attain the spiritual sky in his original spiritual body and engage in transcendental loving service to the Lord in Vaikuṇṭhaloka or Kṛṣṇaloka. When the soul is situated in his original constitutional position of existence, he is said to be liberated. It is possible to engage in transcendental loving service to the Lord and become jīvan-mukta, a liberated soul, even while in the material body.
(10) Āśraya: the Transcendence, the summum bonum, from whom everything emanates, upon whom everything rests, and in whom everything merges after annihilation. He is the source and support of all. The āśraya is also called the Supreme Brahman, as in the Vedānta-sūtra (athāto brahma jijñāsā, janmādy asya yataḥ (S.B.1:1:1). Śrīmad-Bhāgavatam especially describes this Supreme Brahman as the āśraya. Śrī Kṛṣṇa is this āśraya, and therefore the greatest necessity of life is to study the science of Kṛṣṇa.
Śrīmad-Bhāgavatam accepts Śrī Kṛṣṇa as the shelter of all manifestations because Lord Kṛṣṇa, the Supreme Personality of Godhead, is the ultimate source of everything, the supreme goal of all.
(Sri Caitanya Caritamrta------1:2:91-91------purport).