Planning electrical systems to be future-proof

What do you have on the network in your technical room?

Heat pump, photovoltaics, ventilation, electrical distribution 3x, UPS for electrical distribution and the actual server rack is not included yet, although that is located somewhere else. In the technical room, you can also do fixed wiring, i.e. run sufficiently long patch cables directly to the devices.

Choose a larger patch panel and if yes, does that have other disadvantages and how much does it cost approximately more?

24 ports is the limit anyway. That means take a 48-port switch and attach a 24-port patch panel on top and bottom each, connecting them with very short patch cables. When budgeting is that tight, the router is often forgotten as well. Definitely use patch panels with keystones. Additional costs are effectively about 30 euros for the patch panel + keystones + the hassle factor of your general contractor or electrician.

Oh dear, it's slowly getting out of hand. :D
, as an expert, has occupied 23 spots. We are now already (significantly) over that, so one patch panel is not enough and the switch has to be twice as big.

4. 24 will be enough according to your plan.

I come to 24 if I go down from 2 to 1 in the parents' bedroom, but...

For example, I have at the network in the HAR:

...with the stuff in the technical room, I'm clearly over again, or do they only have to be connected to the 48-port switch and not to a 24-port patch panel?

We are currently dealing with the same topics/problems with our floor plan and have ultimately concluded that we will generally install double sockets almost everywhere and rather one more, as it hardly costs more and you cannot change it later. Although we will probably end up with 28 cables, we will only use a 24-port patch panel and a 24-port switch. Only 16 cables are firmly planned, which is sufficient for now. The spare cables will initially remain neatly coiled behind the patch panel. And if the switch is no longer sufficient in 5 years, unlike the cables, it can simply be replaced and the used parts can be sold on eBay.

..with the stuff in the tech room, am I already way over the limit, or do they only need to go to the 48-port switch and not to a 24-port patch panel?

In the tech room, you can also do fixed cabling, i.e. run sufficiently long patch cables directly to the devices.

That was the statement. Of course, you can also connect directly to the switch.

If more than 24 cables arrive in the network cabinet, you simply install 2 rows of the "Keystone holders". It's nothing more than a sheet metal where the modules are clipped in. The rest just remains empty. I think it's dumb to just leave the cables behind dead, because when you need them and start fiddling around.

If you have more than 24 ports, it doesn't necessarily have to be a 48 port switch. You just patch the sockets you need.

Double socket simply means using a duplex cable for the electrician in practice

Electricians are usually mostly "interested" only in lighting circuits, often learned back in the Windows 3.11 days, and perceive themselves as technically savvy even when wiring a TAE socket system. They like double sockets to look like multiple power strips and therefore prefer units with only one data cable entry for both outlets. Knowledge of network technology is "something for young people".

Although we will probably end up with 28 lines, we will only have a 24-port patch panel and a 24-port switch.

That is one of the purposes of patch panels, to simply not plug anything into unused sockets ;-)
The use case of relocating the study to another floor because of a third child is too rare to justify making connections (re)plugable. Whoever has everything 100% and statically assigned could often just as well wire everything permanently.