Congressional conferees yesterday were crafting a compromise package for next year's defense authorization bill that would cap the number of MX missiles at 50 and provide $2.75 billion in research funds for the Strategic Defense Initiative (SDI) missile-defense program, according to Hill sources.

The package being considered by the House-Senate conference committee's special-issues panel would also authorize $100 million more than the administration requested for development of the Midgetman mobile missile and permit three tests next year for an antisatellite weapons system, sources said.

A House conferee said the compromise, which was patched together by Sen. John W. Warner (R-Va.) and Rep. Les Aspin (D-Wis.), appears to have wide support on the special-issues panel that includes a third of the conference committee's 39 members. If approved by the panel, the package will go to the full committee for approval.

He cautioned, however, that panel members remain deeply divided on the timing and conditions for resuming the production of chemical weapons, and that those differences could delay the approval of other issues.

Much of yesterday's debate centered on funding of President Reagan's proposed SDI, also known as "Star Wars," program, sources said. Reagan had asked for $3.7 billion for research, but the House cut this to $2.5 billion. The Senate voted nearly $3 billion, giving discretion to the Defense Department on how to apportion it.

The panel appears ready to compromise on $2.75 billion but is split over the authority to distribute it, with Aspin urging congressional controls and Rep. James A. Courter (R-N.J.) insisting that decisions be left to the director of the SDI office, sources said.

On the MX missile, the panel is seeking a middle ground between the House's ceiling of 40 missiles to be deployed in Minuteman silos and the Senate's decision to "pause" at 50 while the Pentagon looks for basing modes that would ensure greater survivability of the missile against a nuclear attack.

According to the compromise proposal, sources said, Congress would freeze deployment at 50 missiles, reflecting the Senate's numerical preference and the House's desire for a cap. The White House had asked for 100 missiles.

The compromise proposal on Midgetman comes closer to the House bill, which authorized $150 million above Reagan's request for the missile while the Senate stuck to the White House bid for $624.5 million in fiscal 1986.

Sources said Warner suggested a $100 million increase for the single-warhead missile, which would be towed between military bases to complicate targeting by potential attackers.

Sen. Pete Wilson (R-Calif.), who believes the Midgetman design is technically flawed, reportedly pushed for a fresh Pentagon appraisal of the weapon. Aspin, the missile's chief backer, has said he would concur with such a study.

The proposal on antisatellite weapons would allow three tests next year, sources said, seeking middle ground between the House, which wants to ban testing as long as the Soviet Union observes a moratorium, and the Senate, which voted to permit tests as long as the president certifies that he is seeking to negotiate an antisatellite treaty with Moscow.