A Senate Armed Services subcommittee yesterday cut by more than half President Reagan's request to buy 48 MX intercontinental ballistic missiles next year, according to congressional sources.

The strategic and theater nuclear forces subcommittee, dominated by pro-defense senators, voted to reduce to 21 the number of new missiles to be acquired and made cuts in the basing-modification program for Minuteman missile silos scheduled to hold the MX.

The MX reduction vote suggests that the controversial missile may face further cutbacks before the full Senate and House, sources said.

Congress last week approved release of $1.5 billion to purchase 21 MX missiles this year, but members said they would hesitate to vote for that many again.

The MX vote came as Senate Armed Services subcommittees concluded initial markups of next year's defense authorization bill.

Sens. Gary Hart (D-Colo.) and Carl Levin (D-Mich.) sought cuts of $1.2 billion and $1.7 billion, respectively, during the closed-door markup, arguing that the administration is trying to spend too much too soon on Reagan's Strategic Defense Initiative for missile-defense research.

Chairman Barry Goldwater (R-Ariz.) directed the subcommittees to prepare three alternative spending plans for next year. One was for zero growth plus inflation, a level approved by the Senate Budget Committee. The other two called for 3 and 4 percent growth, respectively, both below the administration's request for 6 percent growth above inflation.

Goldwater "really favors a 4 or 3 percent growth," one Senate aide said, "but he wants to show what it would mean to go to zero as the Senate Budget Committee wants."

Democratic congressional sources said the subcommittees found "how easy it would be to have zero growth" in the fiscal 1986 Pentagon budget. "Three percent," one added, "would be a snap."

The subcommittee vote for 21 MXs, for example, was to be the number acquired under all three alternatives. The panel voted down a proposal by Hart to delete all MXs and one by Sen. Sam Nunn (D-Ga.) to limit deployment to 40 missiles and hold next year's production to 12.

On the SDI, the so-called "Star Wars" program, the subcommittee approved three different reduction figures. The president's request for $3.7 billion would be cut by $750 million under the zero-growth budget; $300 million at 3 percent and $150 million if the 4 percent growth level is approved.

The president's request before the Senate committee totals $307.1 billion. To reach zero growth after inflation over this year's spending, the committee would have to cut $17.9 billion. The 3 percent growth target would require $9.9 billion in reductions and the 4 percent growth level $6 billion.

Goldwater's plan to have the committee vote to take three alternative spending levels to the Senate floor for a vote has run into opposition from Nunn, the ranking minority member. Nunn, who has said he favors 3 percent, has told Goldwater he will oppose reporting a bill with more than one level of spending, according to Senate aides.

The tactical warfare subcommittee yesterday voted to hold production funds for the Army's Divad battlefield air-defense gun to $150 million under the zero-growth alternative, according to sources. The program has run into problems during testing. Proposed funding for the 3 and 4 percent alternatives could not be learned last night.

The panel also voted to cut 120 M1 tanks from the administration's request of 840, to meet limits of a zero-growth budget.

The 720 level is "what the Army has privately said it wanted," a Senate aide said. Earlier, the subcommittee agreed that the administration's request for 840 tanks would be fully funded at the 4 and 3 percent growth levels.

Early votes to reduce funds for the Stealth advanced strategic bomber and the advanced cruise missile were restored yesterday, sources said.