An imaginative population expert at the authoritative Overseas Development Council (ODC) has some quick pencil work on the Cancun summit. The calculation is that most participants will devote about five days to the affair, including travel, stopovers and two days of formal meetings; in those five days, the population of the planet will increase by one million people--700,000 deaths and 1,700,000 births.

That same net increase will be repeated every succeeding five days-- relentlessly. For how long? Nobody really knows. But long enough, it is generally agreed, to confound almost beyond measure even the most constructive outcome of the international meeting on cooperation and development at the seaside resort of Cancun in Mexico.

You have to stare long and hard at the past population record and future projections even to begin to comprehend the challenge confronting the 22 presidents and prime ministers who have gathered in Cancun.

Consider: in the first quarter of this century, according to ODC figures, world population increased by 360 million; in the second quarter- century, the increase nearly doubled; in the third quarter it was up another 1.5 billion. The increase for the last quarter-century is expected to be 2.2 billion.

By the first quarter of the 21st century, the increase may be down a bit, to around 2.1 billion, and at some point later in the century, worldwide population may even level off: zero growth. But assorted studies differ widely on just when--or at what level. Never mind. The total, even when the population of the world stops growing, will be at least double and quite possibly triple the estimated 4.5 billion people on earth today.

With that many people already competing for the world's wealth and resources, with some 3.4 billion of them in the poorer developing nations, and 900 million classified by the World Bank in the category of "absolute poverty" (which is to say barely existing)--with all that, there would be urgency enough for Cancun's unique assemblage of big and little, rich and poor, medium-sized and middle-income nations. It's a carefully mixed bag: the United States and Bangladesh, France and Guyana, Sweden and Saudi Arabia, Japan and Tanzania--but not the Soviet Union (by its own decision) and not Cuba (by American insistence).

The agenda is loose and technical: food and hunger, trade, energy and international finance are the general topics. The prospective conflicts center as much on procedure as substance. The "developing" countries represented at Cancun are rich only in numbers (around 120) and centered mostly in Africa, Asia, Latin America--the so-called south. Logically they seek some grand and global negotiation of all the world's inequities and imbalances in a way that would give weight to their numerical majority.

With some exceptions (France, most notably), the well-to-do minority of the so-called north, with the Reagan administration loudly in the lead, would rather work through existing institutions, such as the World Bank, the International Monetary Fund or the general agreement on trade and tariffs where, by and large, money talks.

Nobody heading into the meeting can tell you confidently how this issue can be resolved. That's why there will be no final communique. Cancun's accomplishments will be measured in tone and spirit--in the degree, as one veteran international development expert puts it, to which "there is general acceptance of the indivisibility of the problem."

What's meant by that is a general acceptance of the inevitable consequences for the rich--the loss of increasingly valuable markets--if the poor get poorer. But the test will remain in the force, as well as the nature, of the follow-up. "This could be a last echo or a new beginning," says one of the conference planners.

Hopes for the latter rest in large part on the forces that created Cancun.

Population control is not an easy issue for the world's politicians to address head-on. Other than education in family planning, many experts see no clear solution as such. As one ODC briefing paper on Cancun wryly observed, "No participant will call for a rise in death rates to assure food security and alleviate hunger." Still less will anybody be lobbying for abortion or birth control.

Yet few would dispute an ODC briefer's argument that "a poor, hungry people have nothing to lose by embracing strange causes." Population, in other words, is not just an economic problem but a security problem as well. A recent Library of Congress study documents the point that population is exploding most dramatically in all the wrong places: in the poorest countries, in the cities, "in nearly all those developing countries where the United States and its allies have vital security interests."

The extent to which the Cancun summit gives weight to these grim warnings will say a lot about that glittering gathering.