Wow, this is a lot!
What you need to remember here, is it's not about the answer. There are a million answers. What it's about is how you structure your thinking.
That being said, here's a quick example:
Objective: Getting people to live in the city
1) Understand current needs in market
What do people want? Space? Amenities? Cars? Public Transport? etc.
2) Prioritize needs
Figure out the most important needs. First, identify table stakes needs. Then make sure you address these no matter what.
Second, for non-table-stakes items, rank them. Figure out which ones, have the most "elasticity" I.e. for every extra dollar, how many more people do we get? Rank these
3) Identify pairings/synergies
Expand upon step 2 by figuring out what things can be done together. I.e. if we find out green + space + public transport is important, then, obviously, we can kill 3 birds with 1 stone (i.e. over-index on bikes, trams, etc. and limit options for cars). For these pairings, identify new dollar-per-citizen benefit
4) Set constraints and optimize
Figure out 1) How much $ we have and 2) What we're willing to spend per person.
From here, we can figure out the top designs we can afford, to what degree, and build out the city from there.
In order to figure out the # of people, you can literally just take what we selected from $2/3 (in terms of rate) and multiple by how much we're spending on each of them to get toal population. That's the beauty of this structure...you've done two-in-one :)