I hope. How many citizens do I need in order to get new building options?

Right now I've got about 12 huts and 70 citizens, and only building options are build hut, road, well, or clear land. Will more become available or is there some "mystery goal" I'm supposed to be hitting here? Or is this first scenario only about building up a population? Thanks.

You have a scenario goals page. When you meet them, you advance to the next scenario. Buildings are usually unlocked in the next one. I haven't played Pharaoh for a while now, but I don't remember it having buildings unlock during a scenario. That feature was introduced in Caesar 4 if I remember right.

They do. I looked it up somewhere (but forgot the number already). Once you hit a certain population then you have options to build water supplies, bazaars, etc. Until then all you can do is housing and wells.