In a word: none.

More broadly, there is no indication that Michelangelo had a sexual relationship with anyone, as far as I'm aware (although admittedly this is a difficult thing to prove/disprove). The art historian James M. Saslow provides a great discussion of this in his article "'A Veil of Ice between My Heart and the Fire': Michelangelo's Sexual Identity and Early Modern Constructs of Homosexuality" Genders 2 (Summer 1988). Despite Michelangelo being the best documented early modern artist, and despite the survival of hundreds of his poems and letters, there is no suggestion that he was anything other than chaste (although Saslow notes that this record really starts around age 30, and before then it's not as clear).

There are records of other people accusing Michelangelo of inappropriate (at the time) relationships, or suggesting such things took place (e.g., the father of a potential apprentice to Michelangelo offering his son's services in bed as an added incentive for the artist to take him on—which Michelangelo refused), but these sorts of accusations don't necessarily hold up to scrutiny nor are they corroborated by other evidence.

There's also clear evidence, in Michelangelo's own poetry and letters, of his great love/affection for (in particular) a young nobleman named Tommaso de' Cavalieri—but nothing to indicate the relationship was anything more than the artist expressing his feelings via poems, letters, and drawings.

Admittedly, all our sources are biased. Giorgio Vasari and Ascanio Condivi, contemporaries of Michelangelo who both wrote biographies of the artist, claim the artist was chaste. They stress this in particular when talking about the artist's relationship to Cavalieri. But they're also both concerned with deifying the artist, and Condivi's biography was (in the eyes of many) largely dictated by Michelangelo himself (i.e., not literally dictated, but overseen by him). (Interestingly, when Michelangelo's poems were published after his death the male pronouns in many of the more passionate poems were changed to female pronouns—something that wasn't rectified until the 19th century.)

That said, most scholars of Michelangelo acknowledge that the artist was attracted to men and the male form. Looking at his body of pictorial and sculptural works is evidence enough: overwhelmingly, Michelangelo preferred depicting the male form, clothed and unclothed (though preferably the former), even when portraying women (e.g., a male model was very clearly used for the Libyan Sibyl on the Sistine Ceiling). And, as mentioned above, he had very strong feelings for at least one young man (although he also felt similarly for the noblewoman and poet Vittoria Colonna). What all of this means for Michelangelo's "sexuality" is an entirely different (and more difficult and interesting) question, which is what Saslow attempts to unravel in his essay.

In addition to the article above, I'd also recommend, again by James M. Saslow, the article "Michelangelo: Sculpture, Sex, and Gender" in Looking at Italian Renaissance Sculpture, ed. Sarah Blake McHam (Cambridge University Press, 1998).

EDIT: fixed the date of the first citation.