Anyone who gives spiritual instructions based on the revealed scriptures is accepted as a spiritual master. Although others give help in showing the way to beginners, the guru who first initiates one with the mahā-mantra is to be known as the initiator, and the saints who give instructions for progressive advancement in Kṛṣṇa consciousness are called instructing spiritual masters. The initiating and instructing spiritual masters are equal and identical manifestations of Kṛṣṇa, although they have different dealings. Their function is to guide the conditioned souls back home, back to Godhead.
A devotee must have only one initiating spiritual master because in the scriptures acceptance of more than one is always forbidden. There is no limit, however, to the number of instructing spiritual masters one may accept. Generally a spiritual master who constantly instructs a disciple in spiritual science becomes his initiating spiritual master later on.
Ref >> Srila Prabhupada Vani.