I thought the WTS puts for the number as 20 billion.
Resurrection During 1,000 Years. The exact number of humans who have ever lived is unknown to humans. However, by way of illustration, if Jehovah resurrects 20 billion(20,000,000,000) people, 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.