Krishna shows this whole universe inside his mouth to mother Yashoda and sage markandeya. What about other innumerable universes? Do they have their own krishnas?
Krishna is the Lord of all universes.
Personified Vedas offer prayers to Lord Maha Vishnu: Bhagavata 10.87.41
dyu-pataya eva te na yayur antam anantatayā tvam api yad-antarāṇḍa-nicayā nanu sāvaraṇāḥ kha iva rajāṁsi vānti vayasā saha yac chrutayas tvayi hi phalanty atan-nirasanena bhavan-nidhanāḥ
(41) Neither the masters of heaven nor even You, can reach the end of Your glories oh Unlimited One, oh You within whom the many universes by the drive of Time, each in their own shell, are blown about in the sky like particles of dust. The s'rutis bearing fruit by [neti neti] eliminating that what is not the Absolute Truth, find in You their ultimate conclusion
Krishna speaks to Uddhava:
saṅkhyānaṁ paramāṇūnāṁ kālena kriyate mayā na tathā me vibhūtīnāṁ sṛjato 'ṇḍāni koṭiśaḥ
I who create the universes by the millions, may count their atoms after a certain time, but not so My opulences
Shree Krishna is Vishnu itself and Krishna is not lord of universe
The universe itself is part of Krishna (Vishnu) the supreme God The bharaman originate from Krishna and their are infinite bharaman but the Krishna is one and all of them exist inside Krishna
The whole universe is creation of The supreme
OM! That (the Invisible-Absolute) is whole; whole is this (the visible phenomenal); from the Invisible Whole comes forth the visible whole. Though the visible whole has come out from that Invisible Whole, yet the Whole remains unaltered.
OM! PEACE! PEACE! PEACE! Isha Upnishad
The whole (universe) from the invisible whole (Vishnu)
Mundaka Upanishad 3.2.9 says
स यो ह वै तत् परमं ब्रह्म वेद ब्रह्मैव भवति नास्याब्रह्मवित्कुले भवति । तरति शोकं तरति पाप्मानं गुहाग्रन्थिभ्यो विमुक्तोऽमृतो भवति ॥ ९ ॥
sa yo ha vai tat paramaṃ brahma veda brahmaiva bhavati nāsyābrahmavitkule bhavati | tarati śokaṃ tarati pāpmānaṃ guhāgranthibhyo vimukto'mṛto bhavati || 9 ||
He who knows that highest Brahman becomes even Brahman; and in his line, none who knows not the Brahman will be born. He crosses grief and virtue and vice and being freed from the knot of the heart, becomes immortal.
So this rule applies to all Human beings for all times and all places. Sri Krishna was one among so many enlightened souls of that era. There were many enlightened souls much before him also and in our era also.
Hence, this applies to other worlds also, if they exist. What Sri Krishna could do, can be done by all enlightened souls of all times at all places.
However, we have to remember that not every such incident will be recorded or known to world, but very few will be known for the reasons better known to the Almighty God.
Sri Krishna said in BG as follows:
इमं विवस्वते योगं प्रोक्तवानहमव्ययम्।
विवस्वान् मनवे प्राह मनुरिक्ष्वाकवेऽब्रवीत्।।4.1।।
The Bhagavat said This changeless Yoga I had properly taught thus to Vivasvat; Vivasvat correctly told it ot Manu; and Manu declared to Iksvaku.
Similar statement can be found in Rig Veda also.(RV IV.26.1)
I WAS aforetime Manu, I was Sūrya: I am the sage Kakṣīvān, holy singer. Kutsa the son of Ārjuni I master. I am the sapient Uśanā behold me.
Sri Krishna showed Viswarupa to Arjuna and to his mother Yashoda. Similar incident was noted in the Guru Charitra (Page 25), eulogisation of Sri Dattatreya.
Sripada Srivallabha, an incarnation of Sri Dattatreya, shows Viswarupa to his mother.
Then his mother said, "Lord, your maya has deluded me into looking upon you chiefly as my son. In fact, the whole cosmic egg is within your being. That you are my son is but a convention. You should keep my understanding clear of this delusion".
Then the Lord granted her the vision of his real, divine, form. The handsome, tender, lad at once appeared radiant like a million suns and pleasing as a myriad moons. His blissful nature seemed to enliven every atom of existence.
"Mother, you ever contemplate this form you now see. You shall soon transcend the ocean of this illusory, phenomenal existence and attain eternal unity with me. These two sons of yours shall live to their full span of life and serve you with devotion. They shall be learned and wealthy and you shall be blessed with grand children and great grand children. They too shall win fair renown", he said.