Is one router enough for the whole house?

Of course one router is enough. I fear that the thread starter actually means something else or has no idea what a router is. Run LAN cabling into all the rooms and, if necessary, install an access point on them. Separate via VLAN. If you have no plan, the solution with two connections and an all-in-one device each is more convenient. Everything else makes no sense in a new building. Especially no repeaters.

Basically, you should always use only 1 (NAT) router for network access (including firewall). Internally, you generally only use L2 switches. Possibly, for security reasons, you can separate the in-house WLAN from the internal LAN with a 2nd router.

The answer was already given in the first post. The discussion about the router term, VLANs, L2 switches, etc. does not get the OP any further.

Our low-end solution works: FritzBox connects to the internet and sets up a WLAN in our living area - would be the ground floor in your case. LAN cables laid at least twice in every room (except bathrooms). A central patch panel and sufficiently large switches are in the entrance of our house unit - flush-mounted in the wall directly under the subdistribution. The guys each have a small TP-Link Nano WLAN router for their own WLAN. In the technical room there is another TP-Link Nano that supplies internet to two of the terraces. Advantage: WLAN is available where needed, installation effort is minimal, when the guys have guests, they connect to their network and not ours - but of course use the same internet connection. Disadvantage: When switching zones in the house, you have to log in to a different WLAN again. This is not relevant to us, the worst that can happen is that a streaming transmission breaks off on the way to one of the terraces, which is not a disaster for us. Costs: LAN cabling with patch panel and sockets in the house (I no longer know) FritzBox, 3x TP Link Nano, 2x TP Link 8-Port Switch < 250€.

However, the networks are not separated. Pure access points would be more sensible for the guys.

Because I just saw another post with photos of Heidi, I assume that it is almost finished. If no LAN cable is laid to the [Einliegerwohnung], testing whether WLAN reaches there is the only option anyway. Whether with or without repeater. In the course of the network separation, a separate connection would be the better solution.