The number of Jews leaving the Soviet Union this year could total between 45,000 and 50,000 - the highest number history - if the present emigration rate continues, a U.S. government report said yesterday.

That estimate is contained in President Carter's semiannual report on implementation of the 1975 Helsinki accords signed at the conclusion of the Conference of Security and Cooperation in Europe.

The accords, signed by almost all the countries of Europe as well as the United States and Canada, cover pledges to seek peaceful relations with each other, to respect human rights and to allow a freer exchange of people, information and ideas.

The report's finding on emigration of Soviet Jews is important because it bears directly on increased U.S.-Soviet trade. Under existing legislation, the granting of most-favored-nation trading status to communist states such as the Soviet Union is tied to that country's policies in allowing free emigration by its citizents.

During 1978, the report notes, 30,000 Jews were to leave the Soviet Union - the highest number since 1973. In citing this year's continuing high rate, it adds:

"Most of those emigrating are first time applicants, but some whose applicants have long been denied - the so-called 'refuseniks' - have been allowed to go."

In other areas, though, the report finds the compliance record of the Soviet Union and its allies "mixed." It is especially critical of the East European communist states for continued violations of the rights of dissident citizens.

The report also given lengthy attention to charges that the United States has failed to live up to the principles of Hilsinki, particularly by failing to eradicate poverty and unemployment and discriminating against minorities and women.

In response, the report points out that Carter sent a memorandum last Dec. 6 to the heads of more than 20 federal agencies calling for a maximum effort to comply withe the spirit if Helsinki. It concludes that the U.S. performance while not perfect, is among the best of all the signatory nations.