The Soviet Union for the first time has agreed to tell the United State on a regular basis how many strategic weapons it has deployed, administration officials disclosed yesterday.

The Soviet commitment is written into the draft of the strategic arms limitation treaty (SALT II) which President Carter hopes to sign soon and send to the Senate for approval.

Administration officials said the Soviet disclosures about their own arsenal should help reassure senators who are worried that the Soviets might cheat by deploying more weaponry than the treaty would allow.

One senator, who recently was bridfed on the disclosure provisions of SALT II, said he indeed felt reassured and was now more inclined to vote for the treaty. He declined to be identified, but is in the Senate's political center -- the constituency Carter must win over to achieve approval.

Under the SALT I treaty, administration officials said, the Soviets would only disclose additions and subtractions to their force of strategic missile submarines, intercontinental ballistic missiles and bombers. The total number of weapons in each category was not revealed, meaning the United States would hav to make an estimate through spy satellites and other means.

Under the proposed SALT II agreement, officials said, the Soviets would give a breakdown of their strategic weapons covered by the treaty, including not only the total number of land missiles but the number of individual warheads each carried. The additions and subtractions of weaponry, sources said, would still be provided every six months as well.

The United States, if SALT II is ratified, will still make its own estimates by every means possible, administration officials stressed, but having the Soviets' own figures will make the job easier and the estimates more reliable.

"It's an important step," said a U.S. official who serves on the SALT delegation.

Former Soviet negotiator V. S. Semenov told Paul Warnke, his American counterpart, that in negotiating the disclosure provisions of SALT II, "You have just succeeded in overturning 400 years of Russian history."

Officials said the United States and Soviet Union would have to furnish each other with detailed descriptions of the strategic weapons covered. Under the draft treaty, each superpower would limit its long-range missile launchers -- whether on the ground, on airplanes or inside submarines -- to 2,250. Of that total, only 1,320 launchers could by armed with MIRVed missiles, those with a bunch of H-bombs inside.

The prospective loss of Iran as an observation and listening post to keep track of Soviet weapons development puts additional pressure on Carter to assure Congress that the provisions of SALT II could still be verified. The draft language requiring a rundown on strategic weapons every six months, plus another provision requiring advance notice of some ICBM tests, should help ease that pressure, according to administration officials.

Although making progress on the verification front, the administration is now under attack from another quarter. SALT II critics on both the left and right of the political spectrum are complaining that too many weapons are left uncovered by the agreement.

Sen. Mark Hatfield (R-Ore.), a liberal, registered that complaint over the weekend, and two House conservatives, Reps. Jack Kemp (R-N.Y.) and Bill Chappell Jr. (D-Fla.), have scheduled a press conference for today to level the same criticism.

Kemp and Chappell are expected to cite a new Library of Congress study by analyst Robert G. Bell in making their case against SALT II.

Bell, after discussing weapons allowed under SALT II, states in his 24-page report that both the United States and Soviet Union might be "less inclined" in the future than they are today to dismantle weapons to conform to the limits of a future arms control treaty, SALT III

On the other hand, he says, modernization allowed under SALT II might make it easier under a SALT III treaty to impose "more stringent prohibitions" against introducing new types of strategic weapons.

"To the extent that" the United States and Soviet Union strengthen their defenses against the new strategic weapons allowed under SALT II, said the report, the task of imposing restraints on those new defenses "will be greatly cemplicated." The report said the same would be true of SALT II theater weapons like the U.S. Pershing II missile and Sviet SS20 medium range mobile missile.