Ans. There is but one Guru. The ten that we know, indicate completeness just as the number 10 is considered a complete numeral. All accounting chores can be accomplished through this number.
Guru Nanak took different (physical) forms. Mathura Bhatt in his Sawaiyya in praise of the fifth Nanak, Guru Arjan Dev, explicitly points out:
ਜੋਤਿ ਰੂਪਿ ਹਰਿ ਆਪਿ ਗੁਰੂ ਨਾਨਕੁ ਕਹਾਯਉ॥ ਤਾ ਤੇ ਅੰਗਦੁ ਭਯਉ ਤਤ ਸਿਉ ਤਤੁ ਮਿਲਾਯਉ॥
(SGGS, P. 1408)
Which means: The Divine light of the Lord Himself assumed Guru Nanak’s form; from him (Guru Nanak) was manifested Angad by merger of quintessence.
The Gurus came (to this world) to prepare us to take the responsibility of “Shabad” or the Divine Word. When the nation became ready to take this responsibility they withdrew themselves. Those who talk of transformation or evolution in Sikhism, are attempting to damage its basic tenets.
Mohsin Fani, who was a contemporary of the sixth Master, Guru Har Gobind, writes, “Sikhs consider all their Gurus as one (spirit)”.