Congressional conferees agreed yesterday to set a statutory limit of 50 MX missiles, according to Capitol Hill aides, approving half the number President Reagan sought for a program he has called the centerpiece of his defense buildup.

The MX agreement by the House-Senate conference committee was part of a package authorizing defense expenditures in fiscal 1986 of $2.75 billion to research the president's "Star Wars" missile defense system and $100 million more than the White House had requested for development of the small, mobile Midgetman missile.

Conferees also approved three tests of antisatellite weapons next year but remained deadlocked on the conditions for modernizing chemical weapons, aides said.

In its MX decision, the 39-member conference agreed to cap at 50 the number of MX missiles deployed in Minuteman silos, authorizing $1.7 billion for construction of up to 12 missiles next year and approving between 12 and 21 in fiscal 1987, aides said.

The House had voted to kill funding for any additional missiles next year and to cap the number at 40; the Senate approved a "pause" at 50 while the Pentagon sought better ways of basing the weapon.

Although the conferees left open the possibility of future deployment of the MX in silos less vulnerable than Minuteman's, most congressional sources believe the administration, which had long pressed for 100 of the missiles, will look for new approaches to strengthen the U.S. land-based deterrent.

On Star Wars, formally the Strategic Defense Initiative, conferees reportedly found a middle ground between the House's approval of $2.5 billion and the Senate's $2.9 billion. The compromise of $2.75 billion is about $1 billion less than Reagan sought for research funds next year but almost double last year's appropriation.

House conferees retreated from their efforts to earmark SDI funds for specific projects, aides said, accepting the Senate position of letting Defense Department specialists decide how to spend the money.

But, aides said, the conference went far to accommodate the House on Midgetman, authorizing $100 million above the White House request for development of the single-warhead missile, which would be towed between military bases. The House voted for $150 million more, while the Senate had stuck to Reagan's bid for $624.5 million.

Aides said a compromise approved for antisatellite weapons would allow three tests next year, but conferees want the president to certify that he is seeking to negotiate an antisatellite treaty with Moscow.

The House had voted to ban testing of the controversial F15 rocket-launched system as long as the Soviet Union observed a similar moratorium, while the Senate wanted to permit tests as long as the president certified he was working for a treaty.