President Reagan, seeking to generate support in Congress for the MX missile and at the same time show flexibility on his defense buildup, today will tell Senate Budget Commmittee Republicans that he can trim $8 billion to $10 billion from his military spending proposals during the next five years, administration sources said yesterday.

Most of the money would come from a less costly plan for deployment of the MX, which Reagan is to propose to Congress soon. The administration's new plan, its third, is due to be unveiled next Monday by a special presidential commission. It calls for placing 100 missiles in improved Minuteman silos, which are said to be cheaper than the new silos that were envisioned in the "Dense Pack" basing plan rejected by Congress in December.

Administration officials expressed confidence yesterday that this MX vote could be turned around, although the White House count at this point still falls slightly below what would be needed if every House member voted. Officials said that 70 Democrats are expected to support the MX and that 30 Republicans are opposed to it, leaving the administration about a dozen votes short of a majority.

The administration expectation is that new support will be created for the MX by the report next week of the bipartisan Commission on Strategic Forces, which has consulted extensively with Congress in the course of preparing its recommendations.

However, Reagan appears to face a much more difficult task in convincing Capitol Hill that the defense budget savings provided by the new MX basing proposal are a sufficient demonstration of the "flexibility" the president suggested in a letter last month to Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) and a subsequent interview.

Administration sources said that the probable savings in the 1984 defense budget will be only $2 billion in a military spending proposal totaling $239 billion, or about eight-tenths of 1 percent.

The president is expected to make this proposal at the White House this afternoon to Senate Budget Committee Republicans who have sought to hold his military budget increase to 5 percent after inflation. Reagan proposed a 10 percent increase. The House has voted about 4 percent.

While Reagan publicly has stuck to his original budget proposal, he has made it known to aides and members of Congress that his No. 1 priority is the MX and that he is willing to compromise in order to get some deployment of the controversial land-based intercontinental ballistic missile.

"It's a little like Social Security," a top administration official said yesterday. "This report by the MX commission is bipartisan and will reflect the views of all sides."

It is so bipartisan, in the White House view, that Reagan plans to let the commission have the spotlight all by itself next Monday when the report is formally presented to him. Tentative plans call for the president to make a statement on it the following day, April 12, either in a speech or a message to Congress.

The administration hope is that Democratic members of the commission or consultants to it will carry the brunt of the selling job in behalf of the new MX plan.

One official quoted Harold Brown, defense secretary under President Carter and a consultant to the commission, as telling congressmen at a recent breakfast that "if the MX is withdrawn or Congress votes it out of existence it would be the first nuclear strategic defeat for any country since World War II."

The administration is prepared to make similar arguments on national security themes if necessary to win approval of the MX plan. "This is important not because it's a bargaining chip in nuclear arms negotiations; it's important to the survival of the country," one official said.

At the same time the administration will attempt to show real flexibility on the number of MX missiles that will be deployed. The prevailing view in the White House is that any deployment will maintain the land-based leg of the U.S. defense "triad" and could be expanded later.

Part of the commission proposal next week will be a call for accelerated research and development of a smaller missile, which would be a more difficult target to hit than the MX.