President Reagan will meet today with his top-level National Security Council to consider changes in the U.S. position at the Strategic Arms Reduction Talks (START) with the Soviet Union.

The key shift under consideration is how to change the current U.S. proposal that both sides reduce their existing forces to 850 missiles each.

One option involves increasing this 850-missile level somewhat. Another option, however, is to drop the missile level completely and have the United States rely instead on the number of missile warheads in each arsenal as the main measure.

Either way, the idea is to make room in the future missile count for a shift by both the United States and the Soviet Union away from big, multiple-warhead missiles to a new breed of smaller, single-warhead missiles that would be less threatening and therefore less tempting to fire in a crisis.

After meeting with the National Security Council, composed of top Cabinet officers dealing with security issues, Reagan also is scheduled to meet with congressional leaders, officials said, to explain his approach to the Geneva talks and his implementation of the recommendations of the recent special commission on U.S. strategic nuclear forces headed by retired Gen. Brent Scowcroft.

The president, some officials said, may withhold his final decisions until after Congress votes on the new, multiple-warhead MX missile, which Reagan also wants approved.

During the past two weeks, Reagan has been pressed by a number of congressmen to respond to the Scowcroft panel's recommendation that the White House "reassess" its START proposal to bring it into line with the commission's findings.

The present U.S. START proposal calls for the United States and the Soviet Union to reduce their nuclear warheads to 5,000 each, down from about 7,500 today, and to limit the numbers of missiles to 850 each. The Soviets now have about 2,350 land- and submarine-based missiles, while the United States has about 1,600.

The Scowcroft commission pointed out that this ratio of warheads to missiles still leaves too many warheads that could be fired at each missile silo. The commission called on the administration to change or eliminate these missile numbers, because they provide an incentive to put as many warheads as possible on a single missile and thus detract from any move to less threatening single-warhead missiles.

The Scowcroft commission and roughly a dozen lawmakers who have written to Reagan about this are strong supporters of developing a small missile for the 1990s that eventually would succeed the big 10-warhead MX missile to be deployed in 1986. These lawmakers are all skeptical about how enthusiastic the administration is for the small missile program.

Reagan is expected to reaffirm his endorsement of the program, which is expected to have about $600 million allotted to it this year, officials said. But the president also will insist, as Scowcroft recommended, that Congress approve deployment first of 100 MX missiles.

In other possible changes to the START proposal, officials said there could be some future shift in the currently proposed level of 5,000 warheads on each side and some greater emphasis on limiting and equalizing the overall lifting power, called throw-weight, of each side's missiles. The Soviets have a substantial lead by that measurement.

A key issue is whether the administration will take this opportunity to make a major reevaluation of other aspects of the proposal.

Administration officials said they believe Reagan will discuss other requests from the lawmakers, but may not take formal action on them now. This includes a request that a new commission be created to advise Reagan on arms control and that a so-called "build-down" proposal be incorporated in the START proposal.

The build-down idea, in which each country would remove two older atomic warheads for each new one added, is viewed as more likely to gain some ultimate White House support than the commission idea.