There's also the issue of coverage and the definition of it. Most people don't realise that single-router coverage will fall back on the 2.4Ghz band for most areas of the house, with much lower throughputs compared to the 5Ghz band. Simple physics - the 5Ghz frequency has lower penetrative power and hence is more disrupted by your walls, furniture, etc. Do you just want coverage, or do you want acceptable surfing speeds even at the ends of the home with all doors up? Again, the definition of acceptable differs.
You will cover most flats using 802.11n on 2.4Ghz - it will be slow, and it will be old tech. You don't really need to be spending on a modern router if you're just bothered with 802.11n type coverage to meet your coverage needs. 802.11n was fully ratified in 2009, so any router from the last 13 years will do if that's all it takes to satisfy your coverage requirements. Note that on 2x2, 40Mhz, 802.11n has a maximum theoretical throughput of 300Mbps. Real world use will be typically 70% of that, and as you go further away from the router with more obstacles in between, it drops due to interference. Your ends of the home could be getting 54Mbps type speeds (again theoretical max, so 70% to get real world) or lower.
It falls back to
what you want and expect from your network. If you want full 5Ghz coverage (to get your approx. 400-800Mbps real-world speeds even in the furthest parts of your home, then you will need to invest in more APs and site them accordingly.
Then the kicker: we're dealing with BITS here. The files we download are measured in BYTES, so divide further by eight (1 byte = 8 bits) and you can see file download speeds plummet still further. So suddenly my minimum 400Mbps coverage is in reality only 80 Megabytes per second - which is perfectly acceptable browsing and downloading speeds even when I'm in my room with the doors closed for zoom and all (or for some, with their aircon on).
For those of us who work in our rooms with our doors closed (because the doors also become another obstacle to penetrate), then you will see why.
Again, as an illustration,
@Mach3.2's excellent heatmap of his typical BTO 5-room layout on the 5Ghz band best illustrates the effect of how the walls and doors can kill signals. One AP will not suffice even if centrally located because the walls and doors alone will kill signals. The structural wall itself will kill signals.
This does not apply if you don't care for usable throughput and just want to fall back on "coverage" as sufficient barometer, but I won't be surprised if they struggle to penetrate structural walls too.