How many individuals could there be on earth?
The question of how many individuals there may be in a generation certainly depends on the civilization, on how things are done. Often - in terms of all people, not just one generation - this is discussed in terms of the carrying capacity (click here for Wikipedia article with several references, or refer to population growth for thoughts about how many individuals there could be at any one time).
Assuming individuals somewhat similar to humans, the number of individuals in one generation may be in the order of a billion people, maybe 100 million or a bit less, maybe 10 billion or a little more. The total number of people on the planet at any time will be higher, as several generations overlap.
One can also add time. How many generations could there be? Unless someone finds a way to move it, our planet will become uninhabitable at the latest in about 5 billion years. That would leave room for millions of generations, about two hundered million, at the current length of a human generation.
The number of individuals on this planet might thus be in the order of a hundred million billion or 10 to the 17th (100 000 000 000 000 000). Alternatively, if life of the individuals gets longer, there could be more life-years. Increasing the life span of individuals on the planet would have to increase the generational time and thus necessarily reduce the number of individuals, but increase the number of life-years. The marginal utility of these extra years may reduce in time, thus longer life span would probably lead to less total happiness. What is more, with increasing or even infinite life-span, it would be more difficult to maintain reproductive capacity to replace the individuals that eventually do pass away (e.g., in accidents). With a necessarily low fertility, a very long life span would seem a rather instable system.
Either way, time matters. The number of individuals or life-years would have about seven zeros less if civilization can not find a way to continue. Reducing the assumed number of individuals alive in one generations from 1 billion (people in India) to 10 million (Delhi) only takes away two zeros, increasing it from 1 billion to 10 billion only adds one.
Managing to move elsewhere would seem to increase the number of individuals anywhere considerably, as the number of potentially suitable planets in the milky way galaxy might itself be in the order of many billions. It would probably conflict with the ability of life to develop there in its own right, though, so might not increase the number of possible individuals much.