saṅkhyānaṁ yādavānāṁ kaḥ
lakṣeṇāste sa āhukaḥ
Who can count all the great Yādavas, when among them King Ugrasena alone was accompanied by an entourage of thirty trillion attendants?
30 trillion+ is a very big number, however, the population of Yadavs seems to be quite large at that time:
The members of the Yadu dynasty are virtually innumerable; indeed, just to educate their children the Yadus employed 38,800,000 teachers. - SB 10.90
The Yadu dynasty produced innumerable great men of famous deeds. Even in tens of thousands of years, O King, one could never count them all. - SB 10.90