eta mūrti-bheda kari' kṛṣṇa-sevā kare
kṛṣṇera śeṣatā pāñā 'śeṣa' nāma dhare
eta—so many; mūrti-bheda—different forms; kari'-taking; kṛṣṇa-sevā kare—serves Lord Kṛṣṇa; kṛṣṇera—of Lord Kṛṣṇa; śeṣatā—ultimate end; pāñā—having reached; śeṣa nāma dhare—assumes the name Śeṣa Nāga.
He is thus called Lord Śeṣa, for He has attained the ultimate end of servitude to Kṛṣṇa. He takes many forms for the service of Kṛṣṇa, and thus He serves the Lord.
Link to this page: https://prabhupadabooks.com/cc/adi/5/124