Protesting farmers preparing for a tractor march on Washington this morning rendezvoused in far smaller numbers than initially expected yesterday at staging areas in Chantilly, Va., and Davidsonville, Md.

American Agriculture Movement organizers had said a week ago that 10,000 vehicles and 20,000 farmers demonstrating against low crop prices would assemble near the Washington Monument for a 10 a.m. to 3 p.m. rally.

But a Maryland State Police spokesman said yesterday that only about 200 tractors and trucks had arrived at a field near U.S. Rte. 50 for the drive into Washington. They will be routed from Rte. 50 to New York Avenue to North Capitol Street before taking Columbus Plaza Circle to 1st Street NE and heading to parking areas on Independence avenue between 1st and 23d Street NW.

A Virginia State Police spokesman said that only eight tractors had reached a field near Chantilly High School by midafternoon and that about 200 vehicles were expected today. The tractors heading for the rally from Virginia are expected to travel along Rte. 50 across the Theodore Roosevelt Bridge to Constitution Avenue parking areas between 23d Street NW and 1st Street NE.

Farmers also were to hold protest rallies in about 30 state capitals to dramatize their call for a strike beginning Wednesday if they fail to win higher prices.

The biggest appeared to be in Georgia where President Carter's sister, Gloria Carter Spann, was riding in the second tractor of a 1,000-vehicle caravan heading for Atlanta.

Mrs. Spann told UPI that she did not know if the President - who is spending the weekend at Camp David - know of her participation but said, "I guess he's going to find out."

Other protests, often hampered by bitterly cold weather and snow, were begin held in Colorado, Kansas, Louisiana, South Carolina, Texas and Nebraska.

In Richmond, as many as 500 tractor-driving farmers were expected to converge on Capitol Square, where representatives are to meet with Gov. Mills E. Godwin.

A White House spokesman said that presidential aides would meet with protest leaders today and that "as a farmer himself," the President is well aware of the problems that can arise in this area, and he is sympathetic to the struggle that many farmers are facing."

An Agriculture Department official said that Secretary Bob Bergland would be meeting in the Midwest with farm groups and did not want to "insult" the protesting farmers here by sending a substitute.