If the Atlantic Slave Trade had never happened would there have been a lot less African Americans living in the United States today? If so approximately how many?
The answer to the first part of this question is very clear. There would surely be fewer African Americans in the United States if the Atlantic Slave Trade had never existed. The answer to the second part of the question is much less clear because American history would have been tremendously different if the slave trade had never occurred.
Scholars do not know for certain how many slaves were brought to what is now the United States during the time of the Atlantic Slave Trade. In this link, an acclaimed black historian estimates that only about 388,000 of the millions of slaves taken from Africa were actually brought to the United States. Those slaves, of course, reproduced, and we know (see link below) that there were almost 4 million slaves in the United States to go along with about 475,000 free blacks. This link tells us that less than 1 million people of African immigrant descent live in the United States today. This is partly because immigration from Africa was rare up until the last few decades of the 20th century.
So how many blacks would be in the US had the slave trade never happened? We could argue that only the 1 million or so current immigrants and their descendants would have been living here, in contrast to the actual black population of 45 million. We could speculate that there would have been less racism without the slave trade and so Africans might have wanted to come here voluntarily in greater numbers, but this is just speculation. We could equally imagine that fewer Africans would have wanted to come to a country that was practically all white.
In my view, then, the answer to the second part of your question is unknowable. There might have been 44 million fewer African Americans here today, but the number could have varied greatly in either direction.