Wait, you can add more than one graphic cards in one custom PC?

Yes, it has been possible for years.

Yes, though using this for gaming has gone the way of the dodo as most game titles never optimized for this.
NVlink, the protocol replacing SLI, can only benefit programs such as Blender, CAD, etc.

Oh my word!
Is there like a specific PC case to use to add more GPUs?

Most average Full-ATX cases can work, though most of the time you're limited to using just two GPUs in one setup.
AFAIK something like AMD's EPYC CPUs can allow for greater bandwidth and allow the full use of multiple PCIe16x devices, though that platform is geared more toward servers.

Yes.

Rednec0 answered nicely. You just don't see it often because for day-to-day use or gaming it's pretty much useless (very small and diminishing gains compared to price). However in video editing, rendering - which are applications that are built to use multiple GPU (or CPUs) they basically scale 100%, meaning if i cram 4 GPUs in a PC, it's gonna run almost 4x faster than 1 GPU.

When you move to server / workstations they have cases and boards design to take like, 8~10 GPUs to build one large Rendering / AI box.

Oh my word!
Is there like a specific PC case to use to add more GPUs?

Usually a fat 6U server case will do that job, But it's not only the case, those are equipped with "Backplanes" that has all the PCIE x16 slots for those GPUs.

And yes, it's too expensive for individuals.

do you have a link or a specific name of that brand

Not specifically, But look for a brand named "Tyan".

Just wait for 40 series and se peps selling 3090 under market price hahaha

This is another possibility, yes.

Damn, hate when that happen.

Two at the same time?

Yes, in apps like Blender they are designed to make use of more than 1 GPU ( or CPU, even ) so you will gain performance as you plunk in more hardware.

You can even mix-match GPUs and it will still run faster.

Also if you use it for a long period of time the PC/laptop might overheat (or even explode XD)

That's what thermal throttling is for. And I'm gonna say, if your stuff explodes because you put it in a load then It's more likely the PSU is busted.

Overheating issues are even worse on laptops due to how compressed everything is and that a battery is also located. Also the fan is very small.

It do be that feeling
A feeling I know well.
Sadly, I only got my laptop with the RTX 3000 Professional

wait, prices are dropping?! Perfect!