Is there one Guru or are there many?

The Word is the Master of the entire universe, and the soul the disciple, which, having merged in it, becomes indistinguishable from it. In the sphere of unity and oneness, He in the form of the Word is the Guru of all the universe, but in the sphere of plurality, it is the person in whom the Word is manifest and who is granted the status of a Master that acts as a Guru.

Kabir Sahib says, "I know the secrets of that house and have been authorised by Him." The story of how Guru Nanak, while in the state of deep meditation in the bed of the Veyin river (a large rain-water drain), was authorised to act as a Guru by the Supreme Being, is well known. 

Bhai Gurdas Ji says:

Baba Guru Nanak was born with the gift; thereafter he practised it assiduously.

Pauri 1,  24

The Lord called me to His presence, I praised Him and was given the robe (of the True Master), the nectar of the Name then became my food.

 Majh M1, 150-17

Both Kabir Sahib and Guru Nanak Sahib were Great Masters. 

According to history, Kabir Sahib was born in 1398 A.D. on a full moon day near Banaras and his Light merged in the Eternal Light in 1518 A.D. 

Guru Nanak Sahib incarnated in 1469 A.D. at Talwandi in the Punjab and his Light merged in the Eternal Light in 1539 A.D. at Kartarpur. 

This means that Kabir Sahib manifested 71 years before Guru Nanak Sahib and that they were contemporaries from 1469 A.D. to 1518 A.D. Both of them taught the True Spiritual Path of Surat Shabd Yoga.

Similarly, Shamas-i-Tabriz and Maulana Rumi were contemporaries in Persia from 1207 A.D. to 1247 A.D.

Guru Angad Sahib and Dadu Sahib were contemporaries from 1504 A.D. to 1552 A.D. 

Two other great Saints, Guru Arjan Sahib and Dharam Das, were also contemporaries from 1561 A.D. to 1606 A.D. 

It can be seen, therefore, that there can be several perfect Gurus functioning at the same time. 

But for a person who wishes to become a Gurmukh or a beloved disciple, he must have one Guru only, just as a chaste woman has one husband only and remembers and thinks of him alone. A woman visited by a number of persons is a prostitute.

Know the One, see the One, praise the One; believe in One, select One, and seek the One.

A disciple should have one Master only. The disciple should continue to contemplate on the Form of the Master who initiated him even after the latter leaves this world, and he will succeed. 

The Master, when He initiates a disciple, takes His stand by him. He dwells in his heart. He is the ideal. The disciple will succeed because of this fact. He will gain inner experience. He will lack nothing. A Master never dies. He is imperishable. He is a principle or an ideal, which never dies. He merely gives up the body. 

A disciple may serve and live in the company of another holy man. He should, however, maintain internal connection with the Guru from whom he received initiation, and always fix his attention on His form. He is present in his heart and will look after every one of His disciples. 

If one were to shift from one Master to another, he would never gain anything.