House and Senate conferees agreed yesterday to limit spending for construction of new Metro subway lines to $250 million next year, raising prospects of possible delays in the opening of new stations in Northern Virginia, suburban Maryland and the District of Columbia.

At the same time, the conferees failed to agree on a controversial proposal to reduce the number of passengers allowed to use National Airport. With House and Senate members deadlocked on the issue, the conference committee recessed without setting a date for another session. The conferees are unlikely to resume debate until next week, officials said.

A Metro spokesman said last night that the committee's proposal would disrupt current schedules for expanding the subway service. "We are very much concerned about the impact," said Beverly R. Silverberg, the transit authority's spokesman. "It does mean delays."

Metro officials warned previously that curtailments in federal funds would delay the opening of a rail extension through Northern Virginia to a station at Vienna and would also postpone Red Line service to a planned Wheaton stop in Montgomery County.

In addition, officials have said, plans to build several stations on the long-delayed Green Line in inner-city Northwest neighborhoods might also be jeopardized. The Green Line is designed to run through Anacostia and other low-income District areas to Prince George's County. Its construction has been stalled by a court challenge, controversies and shortages of funds.

Metro officials have not had sufficient time to determine specifically what delays would occur as a result of the conference committee's plan, Silverberg said.

The proposed $250 million appropriation for the 1984 fiscal year, which starts Oct. 1, is $25 million less than the amount approved by the House last month. But it is $20 million more than had been recommended by the Reagan administration and by the Senate. Metro's federal allocations for subway construction totaled $285 million this year and the transit authority had sought $375 million in fiscal 1984.

Nevertheless, an Urban Mass Transportation Administration official disputed Metro officials' contentions yesterday, saying that additional funds would be available from other sources. The administration's aim, he said, is to furnish $275 million in federal construction aid for Metro next year.

The National Airport dispute stems from a proposal by Transportation Secretary Elizabeth Hanford Dole to reduce a federally established ceiling on the number of passengers allowed to use the airport. The ceiling now set for 1985 is 16 million passengers. Dole proposed lowering the limit to 14.8 million. The airport currently handles 13.5 million passengers a year.

The House later voted to bar the suggested decrease in the passenger ceiling. The Senate, however, rejected the House's proposed ban. Members of Congress from the Washington area have supported Dole's proposal.

Supporters of Dole's plan have argued it would lead to increased use of Dulles and Baltimore-Washington International airports. Many members of Congress have opposed possible cuts in service at National, and some airlines also have fought restrictions at National.

The conferees broke off debate after failing to agree on a proposed compromise that would have barred Dole's proposal from taking effect for 90 days. Sen. Mark Andrews (R-N.D.), who suggested the compromise, said it would allow time for further negotiations between the airline industry and the Transportation Department.

The Metro and National Airport issues were included in an $11 billion appropriation measure for transportation programs. Because of the deadlock over the proposed passenger ceiling at National, the conferees held up further action on the appropriation bill.