The National Committee for Adoption said yesterday that despite indications in government surveys that 2 million couples want to adopt children, usually because they are infertile, the number of adoptions each year in the United States is only 142,000 to 160,000.

Moreover, the committee said, three-fifths of the adoptions involve step-parents and relatives taking over legal responsibility for a child already in the family. That leaves only about 50,000 to meet the needs of the 2 million seeking children.

The nonprofit group published its findings in a new "Adoption Factbook," which President William L. Pierce said the organization put together because the federal government stopping compiling detailed adoption statistics about a decade ago.

Based on state surveys in 1982, the committee said there were 141,861 adoptions that year, a figure that may be understated by 18,000 or so, Pierce said. Texas led the nation with 12,176 and California was second at 10,500. For the District of Columbia, the figure was 717, for Virginia 3,037 and for Maryland 1,529.

The report said 91,141 of the 141,861 were classified as "family adoptions," in which, for example, a man legally adopts his new wife's children by a previous marriage, or a cousin or aunt or uncle adopts the children of a family member killed in an auto accident.

Only the remaining 50,720 adoptions were by persons outside the family, the report said. The children came from the 510,000 women under 25 giving birth out of wedlock and from a large pool of about 250,000 children in foster care with special needs -- meaning they were physically or mentally handicapped, had been abused or neglected or were from foreign countries.

Based on state breakdowns, the committee said it appeared that 17,602 of the 50,720 adoptions by unrelated persons outside the family involved healthy infants. Another 14,005 involved children with special needs; 5,707 were adoptions of children from other countries, and 9,591 involved children adopted by their foster parents.

Pierce said the pool of adoptable children would be enormously larger if more of the 510,000 women under 25 who give birth out of wedlock put their children up for adoption. Half these children are white and half are black and 98 percent are classified as healthy infants.

But it is estimated, he said, that 8 percent or less of the white children and 1 percent or less of the black are put up for adoption. Most girls, some in their early teens, keep them and a high proportion go on welfare. One barrier to adoption of these children is the requirement -- in at least 15 states in 1982, according to the report -- that "the cultural and/or racial identity of the child be preserved" or related requirements.

Pierce said that a study by Dr. Christine Bachrach of the National Center for Health Statistics shows that where the child is born out of wedlock, "the young mother does better if she decides on adoption, and the child certainly does better."

According to the report, Bachrach's study shows that among a group of unmarried mothers keeping the baby, the poverty rate was 39.8 percent, compared with 18 percent for those giving it for adoption, and half of those keeping the baby were on welfare, compared with a fifth of those giving the baby for adoption.