The U.S. Army wants to build substantially more Pershing II missiles over the next five years than it needs to put the planned 108 in the field with American troops in West Germany, according to informed sources.

An administration official said last week that, if there is no arms control "breakthrough" in the negotiations with the Soviet Union in Geneva over intermediate-range missiles, "we should not stop at 108" of the controversial Pershing IIs in Europe.

The Pershing II, whose nuclear warheads would be able to hit Soviet targets within eight minutes from bases in West Germany, has become a focal point in the arms race between the United States and Soviet Union, drawing particular criticism from Russian leaders, including Soviet President Leonid Brezhnev.

Plans to deploy the Pershing II plus 464 ground-launched cruise missiles in NATO countries have also drawn protests from the growing peace movement in Western Europe and caused West German Chancellor Helmut Schmidt problems within his Social Democratic party.

Now, according to informed Reagan administration sources, the Army is also planning to build and sell to West Germany a "reduced range" RR version of the Pershing II missile, armed with either a nuclear or conventional warhead.

At the same time, it was learned that the Pentagon has dropped the idea of MIRVing, or putting more than one bomb, on some of the Pershing II missiles, an idea whose feasibility it had been studying for two years. Scientists determined that Pershings loaded down that way could not reach Soviet territory, according to a Reagan administration source.

The reduced-range, or RR version of the Pershing II, sources said, would not be able to reach Soviet territory. It would be a replacement for the 400-mile-range nuclear Pershing now in the hands of the West German Air Force. There are 78 of these; their nuclear warheads are controlled by American troops and can only be released upon the order of an American president.

One reason for building more Pershing IIs, sources said, was the escalating cost of the missile because of unexpected problems encountered in its development. By building more and selling some, the costs can be spread and the price per missile can be forced down.

In a recent report to Congress, the Army said that the cost per missile had risen $61 million to $168.7 million, an increase of 56.8 percent between March and December, 1981.

In order to have enough Pershing IIs for development, testing and then the deployment of 108, as approved by NATO in December, 1979, the Army has ordered 226 missiles, according to documents supplied to Congress.

Those missiles would be purchased with funds authorized in the budgets of fiscal years 1982, 1983, and 1984. In the Pentagon's five-year budget sent to Congress in January, the Army also carried funds to be authorized in the years 1985, 1986 and 1987 for the purchase of additional Pershing II missiles. The exact numbers, however, were classified.

According to information supplied last year to the House Armed Services Committee by Maj. Gen. James P. Maloney, the Army's deputy director of weapons systems, and recently released in a declassified hearing, President Carter in November, 1980, "approved in principle a program of cooperation with the Federal Republic of Germany for the modernization of their Pershing Ia missile system."

Maloney went on to say that "discussions with the German government were initiated in the same month" and were "expected to culminate in the near future with a German decision to deploy Pershing II RR following the U.S. deployment of Pershing II."

No such agreement was reached last year, according to Pentagon sources.

West German officials in Washington refused last week to comment on any future plans to purchase a "reduced range" Pershing II.