As to population maximum, the WTS has speculated that there could be 20 BILLION on the earth.
*** it-2 pp. 792-793 Resurrection ***
Resurrection During 1,000 Years. A very liberal estimate of the number of persons that have ever lived on earth is 20 billion (20,000,000,000). Many students of the subject calculate that not nearly so many have lived. Not all of these, as it has been shown in the foregoing discussion, will receive a resurrection, but even assuming that they did, there would be no problem as to living space and food for them. The land surface of the earth at present is about 148,000,000 sq km (57,000,000 sq mi), or about 14,800,000,000 ha (36,500,000,000 acres). Even allowing half of that to be set aside for other uses, there would be more than a third of a hectare (almost 1 acre) for each person. As to earth’s potential food production, a third of a hectare will actually provide much more than enough food for one person, especially when, as God has demonstrated in the case of the nation of Israel, there is abundance of food as a result of God’s blessing.—1Ki 4:20; Eze 34:27.
On the question of the earth’s food-producing power, the United Nations Food and Agriculture Organization maintains that, with only moderate improvements in agricultural methods, in even the developing areas the earth could easily feed up to nine times the population that scientists have estimated for the year 2000.—Land,FoodandPeople, Rome, 1984, pp. 16, 17.
How, though, could the thousands of millions be adequately cared for, in view of the fact that most of them did not in the past know God and must learn to conform to his laws for them? First, the Bible states that the kingdom of the world becomes "the kingdom of our Lord and of his Christ, and he [rules] as king forever and ever." (Re 11:15) And the Bible principle is that "when there are judgments from you [Jehovah] for the earth, righteousness is what the inhabitants of the productive land will certainly learn." (Isa 26:9) In his due time, when it is necessary to make it known to his servants, God will reveal how he purposes to take care of this work.—Am 3:7.