Is there a Sunnah prayer that reaches six rak'ahs before Fajr, and what is it?

If what is meant is performing six rak'ahs between the Adhan of Fajr and the Fajr prayer, then there is no basis for it. Rather, performing supererogatory prayers of more than two rak'ahs before the Fajr prayer is disliked by many scholars, and this is what is established from the practice of the Prophet (peace and blessings be upon him).

However, if what is meant is performing six rak'ahs before the Adhan of Fajr, then this is part of Qiyam al-Layl (night prayer), and there is no limit to the maximum number of rak'ahs for Qiyam al-Layl. The entire night is a time for prayer, especially its latter part, which is when the Lord, the Mighty and Exalted, descends to the lowest heaven, and the virtue of Qiyam al-Layl is well-known. It has been authentically reported from Aisha (may Allah be pleased with her) that the Prophet (peace and blessings be upon him) did not exceed eleven rak'ahs, whether in Ramadan or otherwise.

Visions and dreams do not establish Islamic rulings. Rather, Islamic rulings are derived from the Qur'an, the Sunnah, and the explanations of the people of knowledge.