This argument, if anyone is bothering to make it, is factually incorrect. UNICEF estimates that there are about 140 million orphans worldwide, and these are only the ones who have been counted. There are many more children, particularly those who have been kidnapped by sex traffickers, who are not included in this statistic. So, there is no lack of children to adopt. In the United States, the process of adopting a child is complex and time-consuming, which is why many hopeful parents choose children from countries where there are fewer barriers, such as China or Cambodia.
It is also important to note that prospective parents, particularly in the United States, tend to be selective on race and age. Black children are the least likely to be adopted within their own countries. Many parents also prefer to adopt babies or toddlers, which leaves older children in the foster care system, often until adulthood.
There is an ethical argument to make against abortion based on one's personal concept of when life begins. However, the argument that abortion will result in fewer children for prospective parents to adopt is simply not true.