Thousands of volunteers, many of them traveling in helicopters and patrol boats, worked yesterday to bring relief to the mid-Atlantic states ravaged by last week's floods.

With damage estimates climbing in Pennsylvania, Maryland, West Virginia and Virginia, President Reagan declared dozens of disaster areas in the region, and national officials of the American Red Cross said they are almost out of funds.

"The devastation is almost beyond imagination," said Karen Wodarczak, a Red Cross spokeswoman in West Virginia, the state believed to have sustained the most damage. "I can't give you exact figures, but we have spent millions so far."

Federal officials estimated that more than 20,000 families were affected by the floods during the last week. The Red Cross has set up disaster headquarters in Clarksburg, W.Va., and in Roanoke to coordinate the relief operation.

Saying that a final monetary assessment of the flood damage is still weeks away, officials from the Federal Emergency Management Administration (FEMA) were trying yesterday to penetrate some of the areas that were most seriously affected.

In West Virginia, 29 counties have been declared disaster areas, 28 people are confirmed dead and 28 remain missing, according to John L. Price, a spokesman for the governor. He said he expects those figures to change as the water recedes from some rural areas that remain inaccessible.

In Pendleton County, which has been isolated by the flood, National Guard members delivered food and water by helicopter. In other areas of West Virginia, sewers were clogged and telephone communications were often impossible.

"The costs here will be so massive that I don't know if we will ever be able to put out accurate numbers," Price said.

State officials say that damage to public schools alone will exceed $5 million, and that many roads and highways will have to be completely rebuilt.

Virginia officials said yesterday that, beginning Wednesday, they would open at least six centers around the state to receive applications for disaster assistance.

They put damage estimates in the state at more than $650 million so far, but said the figure could rise to $1 billion.

"The numbers are enormous," said George Stoddart, a spokesman for Gov. Charles S. Robb. "And I think they're going to grow."

At least 18 people have died in the flooding and 30 central and western Virginia counties and cities have been declared eligible for federal disaster loans and grants. State officials say they expect more localities to be added to the list during the week.

In Pennsylvania, where six counties have been declared disaster areas, state officials have estimated property damage at more than $36 million. There has been one reported death associated with the flood.

"What we need now is money and more volunteers," said Philip Cogan, a FEMA public affairs officer who is currently working in Pennsylvania. "The relief agencies are doing a great job, but they are working at the limits of their abilities."

Flooding in Maryland caused more than $20 million damage, according to preliminary estimates by state officials. No deaths were reported.

Both the Red Cross and the Salvation Army, each of which devoted a large number of resources to aid the victims of Hurricane Gloria earlier this year, said their emergency funds are running low.

"This has been a frightening year," said Kathy Carroll, a spokeswoman for the Salvation Army in Washington.

Red Cross staff members and volunteers have prepared more than 20,000 meals each day in 36 vans sent to regions affected by the floods, according to a spokesman for the agency.

They also are distributing thousands of kits containing mops, brooms and other cleaning materials, and blankets to families washed out of their homes.

People living in declared disaster areas will be eligible for grants from FEMA, 75 percent of which will be paid by the federal government and 25 percent of which will come from the state and local governments. FEMA also runs programs that include temporary housing for homeless residents.

"The help we are getting is wonderful," said West Virginia state Del. Robert Harman. "But a lot of these things people will never be able to replace. No loan can change all this."

People wishing to donate to the flood victims may write: Emergency Relief Fund, The American Red Cross, P.O. Box 37243, Washington, D.C. 20013; or Flood Relief Fund, c/o The Salvation Army, 503 E St. NW, Washington, D.C. 20001.