Questions How was the Universe created? and What is the source of creation according to Upanishads? talk about many aspects of creation. But my questions are:
According to Sankara there is no creation. There is only vivarta vada - apparent manifestation.
Swami Vivekananda says (Complete Works, V3, pp 12-14, also available here under the heading Lectures and Discourses, sub-heading The Free Soul):
Before going into the practical part, we will take up one more intellectual question. So far the logic is tremendously rigorous. If man reasons, there is no place for him to stand until he comes to this, that there is but One Existence, that everything else is nothing. There is no other way left for rational mankind but to take this view. But how is it that what is infinite, ever perfect, ever blessed, Existence-Knowledge-Bliss Absolute, has come under these delusions? It is the same question that has been asked all the world over. In the vulgar form the question becomes, "How did sin come into this world?" This is the most vulgar and sensuous form of the question, and the other is the most philosophic form, but the answer is the same. The same question has been asked in various grades and fashions, but in its lower forms it finds no solution, because the stories of apples and serpents and women do not give the explanation. In that state, the question is childish, and so is the answer. But the question has assumed very high proportions now: "How did this illusion come?" And the answer is as fine. The answer is that we cannot expect any answer to an impossible question. The very question is impossible in terms. You have no right to ask that question. Why? What is perfection? That which is beyond time, space, and causation — that is perfect. Then you ask how the perfect became imperfect. In logical language the question may be put in this form: "How did that which is beyond causation become caused?" You contradict yourself. You first admit it is beyond causation, and then ask what causes it. This question can only be asked within the limits of causation. As far as time and space and causation extend, so far can this question be asked. But beyond that it will be nonsense to ask it, because the question is illogical. Within time, space, and causation it can never be answered, and what answer may lie beyond these limits can only be known when we have transcended them; therefore the wise will let this question rest. When a man is ill, he devotes himself to curing his disease without insisting that he must first learn how he came to have it.
There is another form of this question, a little lower, but more practical and illustrative: What produced this delusion? Can any reality produce delusion? Certainly not. We see that one delusion produces another, and so on. It is delusion always that produces delusion. It is disease that produces disease, and not health that produces disease. The wave is the same thing as the water, the effect is the cause in another form. The effect is delusion, and therefore the cause must be delusion. What produced this delusion? Another delusion. And so on without beginning. The only question that remains for you to ask is: Does not this break your monism, because you get two existences in the universe, one yourself and the other the delusion? The answer is: Delusion cannot be called an existence. Thousands of dreams come into your life, but do not form any part of your life. Dreams come and go; they have no existence. To call delusion existence will be sophistry. Therefore there is only one individual existence in the universe, ever free, and ever blessed; and that is what you are. This is the last conclusion reached by the Advaitists.
So to answer your question more directly, there is no 'need' for creation. Why did Brahma want to become many? Do you mean Brahman or Brahma? If Brahman, the Upanishads simply say that he became many. The Taittiriya Upanishad (3.1) says:
That from which these beings are born, by which they live after birth and into which they they enter at death--try to know That. That is Brahman.
And the Aitareya Upanishad (1.1.1-2) says:
This universe, my dear, was but the Real (Sat) in the beginning--One only without a second [ekamevadvitiyam]. It thought, 'may I be many, (the Atman) willed, "Let me project world!' So It projected these worlds.
Your next question is not logical. You ask - what will happen if nothing will exist in this universe? If the universe exists, then something exists - the universe implies existence, if not of anything else, of the 5 subtle elements.
Iswara is simply the Nirguna Brahman seen through the lens of maya. Our brains are too small and must think in terms of name and form. We cannot conceive of That which is beyond name and form, so we give the Unmanifest a name and form so that we can try to comprehend It.
What is the relation between the Brahman, the Devas, and human? It is all One simply appearing as many. The devas and humans are all jivas that are caught in the meshes of maya. It is all Brahman. The Chandogya Upanishad says (3.14.1):
All this is verily Brahman.
This answer is from Bhagavad Gita perspective.
What is the need of creation? Why Brahma wants to become many?
Neither need nor any reason to create the universe.
Also Brahman (or Brahma) should be interpreted as in one-ness, whether it is one or many.
BG 9.8 - Keeping "My" nature (prakruti) under control, "I" create [& destroy] the world of beings again & again; which are helpless under the nature (prakruti)
BG 9.9 - And O Arjuna, those actions (Karma-s) don't bind Me, as I am uninterested & detached to them
BG 9.10 - Under "My" supervision, the Prakruti produces moving & non-moving [beings]; With[out] this purpose the world repeats
What will happen if nothing will exist in this universe? Will it affect Iswara?
Indeed the nothingness (or ShunyatA) is the ultimate reality, which many refer as supreme God or Iswara. Hence, we can deduce that there is nothing really existed, exists & will exist.
BG 8.20 - But different from that "unmanifested", is the another eternal "unmanifest" reality, which is not destroyed when all beings are destroyed.
BG 13.13 - I shall speak of that which is knowable. Knowing which, one attains immortality. The supreme Brahman devoted to 'Me', is beginning-less. 'That' is said to be neither existent nor non-existent. (existent = being)
Like, when we are dreaming, so much time passes and things also look real. Only upon waking up, we find that, less time has passed & it was an imagination. Similarly, whatever is manifesting currently for us, looks "real" to us, can be interpreted as being imagined.
This is also the core theme of movie Inception.
BG 9.7 — O son of Kunti, all elementary world becomes "My" nature at end of cycle(Kalpa); Again "I" create them at the beginning of the cycle.
Is there any relation between existence of Supreme Brahma[n], Devas and humans?
The supreme Brahman or "Me" or whatever is the ultimate reality -- is not perceivable as discussed above. Which is depicted as "Nirguna" (beyond formless). The other aspect is "Saguna" (formful or formless). That is the cause of everything existent. It keeps happening as part of Eternal Return:
BG 8.18 - On arrival of day, all manifestations originate from "Unmanifest"; On arrival of night they annihilate into [what is] known as "Unmanifest" only.
BG 8.19 - This [same] elementary world only happens again & again; Annihilates upon arrival of night, [and] originates upon arrival of day.
Mainly Brahman, Deva-s & humans have a relation of time, i.e. "X" duration.
If a consciousness (Purusha) is at the level of:
There can be intermediate planes as well. Refer the section Q-2 in this answer for the scripture proof.
BTW, after humans, there are further lower planes of existence: like animals, insects, bacteria, ... Lower the plane, more the time is spent to realise the supreme reality. Once reached at lowest, again there is a sense of "no time" (i.e. "Me").
If a person has more Sattva mode (goodness), it journeys towards "Me" by going to higher regions than humans; with more Tamas mode (ignorance), one journeys towards "Me" by going to lower regions than humans. The mode of Rajas (passion/desire) only binds to any particular plane.
BG 14.18 - Those with sattva go higher [regions]; those with rajas stay in the middle; those with tamas, having despicable actions, go [further] down.
