Perhaps the eventual goal is billions of people on another planet, I think the medium term goal is just a large enough gene pool to ensure survival of the species. You do not need billions for that. The truth is that even if you want billions you do not necessarily need a whole lot of space for them, if you examine the total area of arable land on earth and how many people you can pack into a city the space requirements are not as large as one might first assume if you have climate controlled growing conditions and a 100% urban population.
I have read a lot of material about the energy required to sustain some sort of colony on Mars but I have never read anything estimating the kind of energy strategy required for terraforming or building a substantial colony. This is why I have doubts about it, I do not see it being properly considered. For example most of the terraforming schemes involve the generation of greenhouse gases, creating a more opaque atmosphere and nullifying the thin atmosphere advantage. So now you have a sun-starved version of earth without the repository of hydrocarbons to dig up and burn off that earth has. I'm no expert though, this is a fairly uneducated opinion.
I have read a lot of material about the energy required to sustain some sort of colony on Mars but I have never read anything estimating the kind of energy strategy required for terraforming or building a substantial colony. This is why I have doubts about it, I do not see it being properly considered. For example most of the terraforming schemes involve the generation of greenhouse gases, creating a more opaque atmosphere and nullifying the thin atmosphere advantage. So now you have a sun-starved version of earth without the repository of hydrocarbons to dig up and burn off that earth has. I'm no expert though, this is a fairly uneducated opinion.