We can never know the exact number for certain because people did not tend to keep specific records of these activities. However, looking at the children of slave women can help us come up with a good estimate. Consider for example Sally Hemmings, who was owned and raped by Thomas Jefferson multiple times. Because these people are more well-known, we can use them as an example. Records for Hemmings tell us that she had 5 children and 2 additional miscarriages. That means Hemmings was impregnated at least 7 times, though the number is likely higher because not all fertilized eggs end up developing far enough for the pregnancy to be noticeable.
There is some disagreement over whether Thomas Jefferson was the rapist in question or whether it was some other male member of his family, but genetic tests have shown several people who are black and share the last name Jefferson are scientifically related to white Jeffersons. So we know it was someone in the family.
Additionally, the practice of Master on Slave rape was so common as to have given rise to names for the percentage of blackness in a person's background (quadroon for someone who was 1/4th black or octaroon for someone who was 1/8th black).
The Thomas Jefferson Foundation notes that at least 4 of Hemmings's children were sometimes identified as white by others.
All this is to say that some scholars have tried to make estimates based on the records and primary sources we have available to us, but most agree that any estimate is destined to fail.