Canto 1 - Boyhood
Bāla-kāṇḍa
Chapter 17: Brahmā Orders the Demigods to Incarnate and Assist Rāma
Text 1.17.29

ईदृशानां प्रसूतानि हरीणां कामरूपिणाम्।
शतं शतसहस्राणि यूथपानां महात्मनाम्॥

īdṛśānāṁ prasūtāni harīṇāṁ kāma-rūpiṇām
śataṁ
śata-sahasrāṇi yūthapānāṁ mahātmanām

īdṛśānām = such; prasūtāni = were born; harīṇām = of monkeys; kāma-rūpiṇām = who could assume any form at will; śatam śata-sahasrāṇi = a billion of; yūthapānām = leaders; mahā-ātmanām = greatly intelligent.

A billion of such greatly intelligent leaders of monkeys who could assume any form at will were born.

The author states the number of the leaders of the monkeys because it is impossible to enumerate the total number of monkeys [who had thus taken birth].