Washington — How do you determine which party wins or loses next week's elections?
Moreover, what will the results mean? How many seats must the Republicans hold in order to claim that voters have told them to ''stay the course''? Or how many should the Democrats pick up in order to claim a mandate for a ''midcourse correction'' to President Reagan's economic policies?
The key numbers to watch:
* In the House of Representatives, if the Republicans lose no more than 12 to 20 seats, the party's leaders will heave a big sigh of relief. More losses than that, and especially a 30-seat or greater loss, would be generally interpreted as a rebuff to Reagan. Democrats and Republican moderates would feel freer to vote as they like in the next session. And a Congress/White House stalemate would loom.
In the 33 Senate races, the Democrats would call holding their own a success, since they have more seats at risk, a 3-to-2 margin. A four-seat Democratic pickup - rated possible, though not probable - would throw the Senate, too, into stalemate the next two years. And the Democrats, with more easy races in 1984, would be on their way back to Senate control.
To prevent loss of Senate control, the White House this final week has adopted ''a Senate strategy,'' as one GOP professional puts it. After an Oct. 26 presidential foray to North Carolina, a state where GOP officials hope to pick up three new House seats to offset losses in that column, Mr. Reagan will close out the week campaigning in four Western states - Montana, New Mexico, Nevada, and Wyoming - with close Senate races.
These Western states have small populations - 1.5 percent of the nation's total - conservative outlooks, and low unemployment. Reagan is not going to California, New York, or Texas - which have 25 percent of the population - with equally close races, but where Republican candidates fear he might rile up and turn out more Democratic than Republican voters.
It is the governors' races, many analysts say, that the fairest rating of the two parties' basic appeal may be registered. Twenty of the 36 governorships at stake are now held by Democrats, so the higher the Democrats register above their base number of 20, the greater the repudiation of Republican leadership will appear.
But it is likely to be the House seat loss total that will get the most attention Nov. 3, the morning after the election.
Publicly, White House spokesmen have been saying a loss level of 30 seats would be normal, so presumably anything below that would not be a ''defeat'' for the President's party.
However, for elected presidents since Eisenhower, the White House party's average loss is only 10 seats the first congressional election - 15 for Ike in 1954, 4 for Kennedy in 1962, 10 for Nixon in 1970, and 12 for Carter in 1978. If Truman's 1950 results are added in, a 27-seat loss for the Democrats that year, the average is 13.6 seats.
It is in the second midterm election for presidents or administrations that White House parties lose big - 47 seats for the Eisenhower Republicans in 1958, 47 seats for the Kennedy-Johnson Democrats in 1966, 43 seats for the Nixon-Ford Republicans in 1974. Also, major military, economic, social, or other events impacted on these second-term elections: recession in 1958, Vietnam and civil-rights backlash in 1966, Watergate and the Nixon pardon in '74. This series of eight-year cycles for an administration's second term, since 1950, averages a 41-seat loss for the White House's party. If losses are counted in all nine congressional elections since 1946 - when the Democrats lost 56 seats in the midst of post World War II economic and political readjustment - the average is 29.
The Reagan White House can hardly label as ''normal'' the average of House seat losses after the major disrupting forces in modern American history, neutral analysts insist.
History is kind to Republicans in one regard: Unemployment has been a poor indicator of House seat losses in the past. The Republicans lost 47 seats in 1958 when unemployment was at its previous postwar high at election time, 7.1 percent. But the Democrats also lost 47 seats in 1966 when unemployment was at its postwar lowest, 3.7 percent. Whether there is a special electoral punch in unemployment at its current double digit level remains to be proved next Tuesday.
As it is, Reagan is campaigning this week at a minimal, scarcely robust level. This means the President's coattails may not prove very useful to Republicans this year.