President Carter is asking Congress for authority to call up as many as 100,000 reservists on his own in an emergency to buttress the Rapid Deployment Force, Pentagon officials said yesterday.

That is twice as many as he has authority to call up now.

Studies of deploying the RDF to such a distant trouble spot as the Persian Gulf have shown that the active duty force might need help from more reserves than are quickly available under current law, Marine Lt. Gen. P. X. Kelley, RDF commander, said at a Pentagon news conference.

As a result, he said, the administration is trying to have the law changed so the president could activate as many as 100,000 reservists for 90 days without going to Congress for approval. Current law limits such a call-up to 50,000 reservists for 90 days.

Carter announced formation of the RDF some months ago as a way of moving troops quickly to trouble spots, such as the unstable and oil-rich Middle East.

Kelley's progress report on the RDF, which he commands from a special headquarters at Tampa, Fla., indicates that it is growing far beyond the originally advertised strength of 110,000 troops to about 300,000, counting reservists.

RDF is an umbrella term for existing units that would fight side-by-side under a unified command. Already tagged for possible assignment to an RDF operation are the Army's 32nd and 101st Airborne division and 24th Mechanized Division, and a Marine amphibious force of about 50,000 men. Those troops, together with associated Air Force and Navy units, could bring the active duty part of the RDF up to a strength of 200,000.

Kelley said some distant emergencies might require sending in as small a unit as a battalion -- about 800 troops -- while others might require the full RDF strength, plus reservists.

Most of the 100,000 reservists sought for quick call-up would come from drilling units on the reserve and National Guard forces of all four military services, not former servicemen going about their civilian pursuits as standby members of the Individual Ready Reserve.

The House Armed Services personnel subcommittee is expected to open hearings next week on Carter's reserve legislation. The Pentagon, in a letter accompanying the proposed reserve bill, said "almost half" of the 50,000 reservists who could be called up under current law would be assisting in airlifting troops to a crisis area.

Authority to activate 100,000 reservists, the Pentagon said, would facilitate a "balanced" call-up. Also, said the Pentagon, planners could count on using some of the 100,000 reservists to fill gaps left by NATO forces that had to be deployed outisde the alliance.

Kelley, who has been running the RDF from Tampa since March 1, said he and fellow commanders, in focusing on the Middle East and Persian Gulf, became convinced that getting there first was vital.

It "changed the whole calculus" of potential conflicts if American forces beat the Soviet military to a crisis area, he said.

In viewing likely trouble spots around the world, Kelley said: "I don't think there is an area in the world as critical to western civilization as the Middle East and Persian Gulf." Losing the oil there would be "absolutely catastrophic," he said.

While Kelley gave his progress report, Marines worked 13-hour days at Wilmington, N.C., loading ships with tanks, artillery, ammunition and amphibious tractors for the RDF. The two ships, Jupiter and Meteor, will be among seven sailing to the island of Diego Garcia in the Indian Ocean next month. They will remain anchored in a lagoon there until called upon for military exercises or to resupply troops of the RDF deployed in a crisis in that distant area of the world.

The U.S. government plans to build 15 ships eventually, specifically designed to carry armor, food and other supplies for the RDF. But seven cilivan cargo ships have been chartered in the meantime by the Military Sealift Command as a stopgap measure.

Navy Secretary Edward Hidalgo and Marine Commandant Robert H. Barrow flew to Wilmington Tuesday to inspect the ships and talk to Marines loading them.

"An incredible performance," said Hidalgo in marveling at the speed and skill in which the Jupiter and Meteor had been loaded with M60 tanks, 155-millimeter howitzers and military vehicles of various sizes.

The fact that the ships will soon be on their way to the Indian Ocean, Hidalgo said, "shows that we can move our frontiers of response forward and sends a very strong signal" to the rest of the world.

Barrow said the seven support ships in the Indian Ocean will be "a way of shortening the response time" for U.S. military forces.

The five other ships to join the Jupiter and Meteor at Diego Garcia are the cargo ship Mercury, the oiler Sealift Pacific, the water tanker Zapata Patriot and the food and ammunition ships American Champion and American Courier. All will be manned by civilian crews.