mjhjmtu

Could you use a 3D printer to make 3D printers parts? What is the most of a 3D printer that can be built this way or parts that break that can't be replaced this way?

Yes, you can print most of the parts (electronics, linear guide rails, ball bearings and nuts and bolts, etc cannot be printed). Actually this was exactly the purpose of RepRap.org:

RepRap is humanity's first general-purpose self-replicating
manufacturing machine.

and:

Since many parts of RepRap are made from plastic and RepRap prints
those parts, RepRap self-replicates by making a kit of itself - a kit
that anyone can assemble given time and materials.

There have been attempts in the past to even replicate the frames of printers (e.g. Dollo 3D or Snappy, but such designs are not very successful, printed frames are more flexible than metal frames.

_{I have built 2 custom printers myself using other printers to print parts and printed all printer parts for several others. It is possible to print your own linear bearings from POM, I prefer these over the noisy metal bearings.}

Yes, printers can print printer parts. With the right design, all non-metal/non-electronic parts can be printed. That is actually the core of the RepRap philosophy.

Some early printers, like the Mendel, did use just materials from home depot and printed joiners together with the electronics.

Can a 3D printer make a 3D printer?

Not without a bunch of other parts

Could you use a 3D printer to make 3D printers parts?

To some extent.

What is the most of a 3D printer that can be built this way

Basically you you can't print the precision mechanical guides, you can't print the electronics, you can't print the parts that need to get hot, you can't print the motors, you can't print the lead-screws or toothed belts. Printing the whole main frame doesn't seem to be very practical either.

Afaict what you are left with is mostly printing the pieces that join all the functional elements of the printer together.