bhaved yugapad utthita
yadi bhah sadrsi sa syad
bhasas tasya mahatmanah
If hundreds of thousands of suns were to rise at once into the sky, their radiance might resemble the effulgence of the Supreme Person in that universal form.
Commentary by Srila Prabhupada
What Arjuna saw was indescribable, yet Sanjaya is trying to give a mental picture of that great revelation to Dhritarashtra. Neither Sanjaya nor Dhritarashtra was present, but Sanjaya, by the grace of Vyasa, could see whatever happened. Thus he now compares the situation, as far as it can be understood, to an imaginable phenomenon (i.e., thousands of suns).
Commentary by Sri Visvanatha Cakravarti Thakur
If at one time the effulgence (bhah) of a thousand suns arose, then it would be somewhat similar to the effulgence (bhasah) of the universal form (mahatmanah).