Actually, here is the quote from the Insight Book:
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.
This was written in the late 80's so yes, the number of "dead resurrected ones" scheduled to come back has grown in the intervening 30+ years.
I don't know where they got the 20 billion from.