Once Lord Brahma is greeted by Krishna’s doorman at one of his palaces and asks:
‘What is your name so I may tell Lord Krishna who has come, desiring his audience.’
‘I am his devotee Brahma,’ was the reply.
Brahma may have wondered how the doorman didn’t recognize him by his four heads, but perhaps this doorkeeper was inexperienced. We assume that many other demigods came to visit Shri Krishna in Dvarka, so perhaps the doorkeeper wanted to make sure he didn’t make a mistake.
After some time, the doorman returned and asked another question to the anxious Brahma, ‘Which Brahma are you’?
With the appearance of the doorman Brahma was expecting to be let in. Thus he was taken back by the question, and puzzled over the meaning. His great intelligence from his four heads was bewildered. What kind of a question was this? As far as he knew he was the only Brahma, so were there others? Brahma is not a man of few words, yet he found himself speechless in confusion. Finally, he came up with a reply.
‘I am the four headed Brahma, the father of the four Kumaras.’
After some time he was escorted in through the incredibly opulent palace to the room where the regal Krishna was sitting comfortably on a royal throne.
Upon seeing Lord Krishna, Brahma offered him obeisances and appropriate worship, and then Shri Krishna honored him with welcoming words. He inquired from Brahma about the nature of his visit.
Brahma replied that he would tell him about the nature of his visit after Krishna helped with a doubt.
Brahma questioned the Lord thus, ‘Why did you ask which Brahma had come? Kindly tell me the purpose of such an inquiry. Are their other Brahmas besides me in this universe?
Krishna smiled at Brahma’s words and called all the Brahmas from the infinite universes. Immediately unlimited Brahmas arrived wth their various numbers of heads. Some had ten, some twenty heads, fifty, a hundred a thousand, some ten thousand, one hundred thousand, some ten million and one hundred million. Their was no counting the number of heads.
Also arriving from each universe were the various manifestations of Lord Shiva with unlimited heads, and King Indra with thousands of eyes on their bodies.
Beholding the unlimited opulenses of Krishna and feeling dwarfed by the manifestation of other Brahmas, our universes Brahma felt like a rabbit amidst many elephants.
Brahma says, ‘There are people who say, ‘I know everything about Krishna.’ Let them think in that way. As far as I am concerned, I do not wish to say very much about this matter. O my Lord, let me say this much. As far your opulences are concerned, they are all beyond the reach of my mind, body and words.’  SB 10.14.38 -39
My dear Kṛṣṇa, I now humbly request permission to leave. Actually, You are the knower and seer of all things. Indeed, You are the Lord of all the universes, and yet I offer this one universe unto You.