President Carter and Defense Secretary Harold Brown have set the administration on a collision course with many military leaders by insisting on quantity over quality from now on in ships, submarines and planes.

"We need numbers," said Brown in spelling out the Carter administration's basic approach to making billion-dollar decisions on new weapons for the Army, Navy, Air Force and Marine Corps.

Declaring in his annual posture statement just released that it will take "imaginative, often courageous decisions" to turn away from the best weapons money can buy, Brown served notice that the administration is determined to try.

Getting specific, Brown in his posture statement called for a medium-sized carrier of 60,000 tons instead of the 90,000-ton, $2 billion Nimitz; a cheaper alternative to the $1.6 billion Trident missile submarine, and something simpler than the advanced Harrier "jump jet" the Marine Corps want s to buy.

As Carter and Brown try to implement those decisions and related smaller ones, may military leaders -- especially retirees -- will protest that the administration is buying second-class weapons.

Besides colliding with the military constituency, the administration's quantity over quality philosophy crashes head-on into that of many influential members of Congress -- including elders on the Armed Services committees who have a lot to say about how much money the Pentagon gets and for what.

From now until fall, the quantity vs. quality argument will be waged in he hearing rooms of the Senate and House. Although Brown and other Pentagon civilians will sit at the witness tables with generals and admirals, they will not always present a united front.

On the aircraft carrier, for example, Adm. Thomas Hayward, the new chief of naval operations, is expected to prt compnay with the administration by recommending a large, conventionally powered ship like the John F. Kennedy already in service over the 60,000-ton "midi."

Brown's posture statement, given to Congress on Thursday, sets forth the administration's case on these major weapons:

Midi carrier. "Construction of this new carrier would mark an essential and important step in reversing the trend of the last decade toward ever larger, more expensive ships. This administration is fully committed to reversing this trend.

"It will be much more capable than any carrier projected to be built" by the Soviet Union or any other country. (The Soviet Union's Kiev class carriers are estimated to be between 40,000 and 54,000 tons -- smaller than the U.S. midi. They carry antisubmarine aircraft which use a short stretch of deck.)

Another Nimitz carrier or a redesigned Kennedy would cost $5 billion to $6 billion more than the midi over 30 years "if the full cost of the aditional aircraft is considered."

(Brown himself, during the White House defense budget reviews, favored an updated Kennedy over the midi, according to Pentagon officials, but is expected to argue strongly for Carter's choice before Congress.)

Trident missile submarine. "Alternative submarine designs potentially less expensive than Trident are under study." If the study is promising, money for an alternative to Trident may be requested in the fiscal 1982 budget.

Advanced Harrier AV8B jump jet. "In light of expected kimitations on funding for procurement of Marine and Navy aircraft in the 1980s, and the need to purchase larger numbers of such aircraft, we have decided to terminate funding for AV8B research and development. While this aircraft does appear to have some potential for Marine Corps close air support missions, it appears that its measurable advantages over a conventional aircraft, such as the dual mission F/A18, may be minimal. In any event, there are advantages in concentrating on fewer types of aircraft."