Good question, i would like to have an answer too

I think they address this with story points. If you want to start off with a good life, you have to have the story points from the previous life. This may only pertain to nobility, but anything that isn't nobility is lowborn?

All of the "lowborn" Families will have Houses.
For those who do not have a House, they would also not have a Family because the only way to begin homeless is to start out as a Ward from an Orphanage.

As to your questions about spread, we only have numbers from the Kickstarter to go off of.
65% of all Kickstarter backers will be coming in no earlier than launch.
Out of those players, the exact number that choose a Family versus a Ward is anyone's guess.

Now, let's take a look at the other 35% players (according to Kickstarter) will be starting as Gentry or above.
It's important to note that these players will not only be taking part in Exposition, but will be the heads of their Households.
Being Gentry does not mean that one is Noble.
In fact, it doesn't even mean that they're an Aristocrat.
62% of Kickstarter backers that pledged Gentry or above are /not/ Aristocracy or Nobility.
That means that 21% of all Kickstarter backers will be the head of a Plebian House.
If you're asking about Nobility (Counts, Dukes and Kings) and not Aristocracy (Barons and Mayors) only 4% of Kickstarter backers purchased a Noble title.
That makes Noble Houses only 14% of all purchased Kickstarter Houses, with 25% of Houses belonging to the Aristocracy (9% of backers starting an Aristocratic House)

A very important thing to note is that these numbers are all based around what was bought during Kickstarter.
Not only does it not take into account new purchases, but it also doesn't take into account the fact that each of the people purchasing these titles will be split up by which server they choose, as well as the actual number of Noble/Aristocracy/Plebian Houses being determined when the world is seeded.
Who knows? Maybe only 60% of the Noble Houses will be PC-run and maybe 40% of Plebian Houses will be run by players. All of these numbers change when Soulbound Studios define precisely what the makeup of titles and Houses are, which I imagine will vary based on Server, Kingdom, Duchy, County and maybe even Settlement.
Time will tell.

Finally, to get to your other questions.

"Will nobles have more children than lowborns?"
I doubt it. Child rate will probably vary based on location and maybe even Tribe, but probably not status.

"Will non-landowners be renters?"
Non-landowners will still belong to a Family unless they are a Ward.
In the Family House (almost every Family has a House of some kind) rooms will be provided for you and your offspring based on your Head of Household providing one.
This Head of Household is considered part of the Gentry.
The Gentry are also not considered Renters as they definitely own their land.
However, they will probably be required to contribute to their County in the form of taxation, just as Counts contribute to the Duchy and Dukes contribute to the Kingdom.
Rental will probably happen in establishments designed for such a purpose, such as a tavern for weary travelers or a market stall in town square.

Remember, counts/nobility will most likely not live alone in their castle/keep/mansion, and a good noble/aristocrat want you to get more children, because more people means more taxes. ((Also I bet a few nobles/aristocrats are actually going to have fewer children of fear of them pushing a claim to the title))

Here's something from an older QnA which might shed some light on this topic:

Q: “Will you as a count/duke/king be able to house other houses or representatives of different families under one home, will the manor/keep/castles if big enough, be able to house them, and how many would be able to live there?”

A: Yeah you can, there’s a zoning system within a house. It’s how when you have children when you designate rooms in your house as belonging to their children with their own locks and their own privacy and that sort of thing. It’s a similar system to how innkeepers work with an inn, where each room can be designated to belong to a different guest. So you can have as many people as you want living in your house, just based on the number of rooms you have.

Edit: It doesn't say anything about you owning the house, but if someone leased out 2 rooms or a small "apartment" in their house to two people and they then dedicated their one room to a child then I'm sure that'll work somewhat... Maybe, only time can really tell.

I think it basically boils down to wealth and how large of a house you have. Each child must have their own bedroom so if you can only afford one extra room, you will be limited to only one child. If your finances support numerous additional bedrooms, then you can have as many as your wallet will support. Now I don't know if you actually have to actually 'own' the home or if a rental will work. I would think it would lean towards ownership, but I am not totally sure on that.