So to briefly answer this, historians cannot answer this entirely because, we honestly don’t know for certain. Whether or not Jefferson had a sexual relationship with his slave, Sally Hemmings was under fierce debate for nearly two centuries after the story broke during Jefferson’s presidency. Probably the most accurate accounting now of that can be found from the Thomas Jefferson Foundation (TJF) Research Committee Report on Thomas Jefferson and Sally Hemings which ultimately concluded:

JF and most historians believe that, years after his wife’s death, Thomas Jefferson was the father of the six children of Sally Hemings mentioned in Jefferson's records, including Beverly, Harriet, Madison, and Eston Hemings.

The only reason people suspected this relationship in the first place was because in 1802, a political journalist named James T. Callender published an account stating that Jefferson, “kept, as his concubine, one of his own slaves”. This was a scandal of course in the 19th century but Jefferson survived the political fallout from it. It is very possible he had sexual contact with other slaves, but there’s no other primary documents that can substantiate it nor is there DNA evidence that has been studied with like Hemmings. But it should be worth asking, how common was sexual contact, whether ‘consensual’ or assault between owners and the people whom they enslaved. It should also be noted that I put ‘consensual’ in quotes because historians have correctly pointed out that to own another person is to be in a position of unquestionable authority, free from repercussions. Thus even if a slave agreed to the sexual contact, they don’t actually have ‘free will’ to make that decision.

Historians have discussed this many times over the last century. Articles like this one actually show quite well how common sexual contact between white male owners and female enslaved women resulted in the birth of illegitamate children, stating, “Sexual abuse of slave women was extremely common, and the victims experienced no justice.”

One of the most earliest, and arguably most significant works about this type of abuse was the narrative account by a former slave named Jarriet Jacobs. Her account, entitled, Incidents in the Life of a Slave Girl was published in 1861, and was the first time that, in print, it accounted for how slave owners sexually abused those whom they enslaved. History would show that this account was far from uncommon, and historians showed, especially throughout the 1970s and 1980s (as original research heavily studied this field) that it was extremely common and that apparently everyone turned a blind eye to it.

The reasons that slave owners did this, apart from the sexual gratification, was to punish and assert control over enslaved women. (Women and Slavery - Boundless Open Textbook. Boundless U.S. History. 2015). But enslaved women also were not viewed as humans, but rather a commodity. As Jacobs recalled in her account, “women are considered of no value, unless they continuously increase their owner’s stock. They are put on a par with animals.”(page 49). This is significant because it then became common for male owners to try and get their slaves pregnant in order to sell the offspring later, if she wasn't already having sex with other enslaved men.

Given this history, it appears very possible that Jefferson could have had sexual contact with other slaves, but there is no evidence to support it, but was unfortunately quite common during this era.

Edit: added a source

It is highly improbable that Jefferson had sexual relations with any other slaves. We actually do have a source for this: The Memoirs of Madison Hemings, Thomas' son with Sally. Madison Hemings' memoirs are a key source about Thomas and Sally's relationship, and also Thomas' relationship with his slaves in general.

In his memoirs, Madison writes:

We were the only children of his by a slave woman.

While it is vaguely possible that Madison was wrong, it seems very unlikely that he wouldn't know if he had other half siblings. The consensus of historians tends to agree with what Madison said here. There is no evidence Thomas Jefferson ever slept with another slave besides Sally Hemings.

For more information on this, I suggest Madison Hemings' memoirs, and also the leading scholarly authority on Jefferson-Hemings: Annette Gordon-Reed. More specifically, Gordon-Reed's book The Hemingses of Monticello is the most direct treatment of the subject.