The 1984 Democratic National Convention will have one-sixth more delegates than the 1980 convention, with southern states gaining strength relative to the rest of the country because of their population growth and good showing for Jimmy Carter in 1976 and 1980.

The convention will have 3,923 delegates, compared with 3,331 in 1980, according to a preliminary apportionment released yesterday. The allocation plan will come up for approval at the next meeting of the Democratic National Committee, Feb. 5.

Most of the increase is the result of the national committee's decision last year to set aside 561 delegate seats for elected and party officials unpledged to any presidential candidate. Additional votes were awarded some states because of population changes, and all states were guaranteed against losses.

The Democrats compensated for the growth in delegate ranks by reducing the number of alternates from 2,053 in 1980 to 1,310 next year. Overall, the convention size will drop from 5,436 to 5,242 people.

There is a slight disparity in the totals because some small states are allowed to send more delegates than they have votes.

According to the proposal, the District of Columbia will have 19 delegates, the same as in 1980, and six alternates; Maryland will have 74 delegates, up from 59 in 1980, and 25 alternates; Virginia will have 78 delegates, up from 64 in 1980, and 26 alternates. In each case, the delegate total includes the unpledged party and elected officials.

The South gained relative strength both because of population increases recorded in the 1980 census and the working of the Democrats' allocation formula.

That formula is based, in part, on the states' votes in the three most recent presidential elections.

This time, the formula did not include 1968, when George C. Wallace ran strongly in the South on his independent, third-party ticket. And it included both 1976 and 1980, when Jimmy Carter ran better in the South than in other parts of the country.

The formula plus the population gains produced spectacular increases in voting strength for some southern states. While the average state delegation increased by just under 18 percent, Florida grew 43 percent, Tennessee 38 percent, Alabama 38 percent, Mississippi 34 percent, Louisiana 33 percent, Texas, 32 percent and Georgia and South Carolina 30 percent each.

The Democrats have not selected the site for the convention, which is expected to be in July, 1984. Republicans will meet the following month in Dallas.