It is impossible to see the millions of other Brahmandas outside our inner universe (inside a massive Brahmanda) because of being surrounded by 7 thick material layers that only mystic yogis and advanced devotees can see through

What we see in this universe is all within our 4 billion mile diameter individual universe, we CANNOT see outside of our small universe. Therefore It is impossible to see the millions of other Brahmanda universes outside our inner universe (that is deep inside a massive Brahmanda) because of being surrounded by 7 thick material layers that only rare mystic yogis and advanced devotees can see and pass through in their subtle material bodies

Srila Prabhupada explains why we cannot see outside our own universe - Srila Prabhupada - ''The Vedic literature informs us that within this universe there is one sun, which is moving. The Western theory that all the luminaries in the sky are different suns is not confirmed in the Vedic literature. Nor can we assume that these luminaries are the suns of other universes, for each universe is covered by various layers of material elements, and therefore although the universes are clustered together, we CANNOT see from one universe to another. In other words, whatever we see is within this one universe. In each universe there is one Lord Brahmā, and there are other demigods on other planets, but there is only one sun''.
- Srimad Bhagavatam Canto 5 Chapter 21 Text 11, Purport: