How high can your camera go in ISO?

I go as high as necessary to be able to take the photo.
Yes, you have limitations, but you have to learn to deal with them, and to work with them.

But of course there are also a lot of misconceptions about noise.

1: As you describe, look at 50% and not at 100%. Because especially when you view photos on phones, tablets (and even laptops), noise is often not distracting, because the photo is not viewed large enough for you to notice the noise. (just look at your example of that flower, when does the noise actually become distracting at this size. Is there a difference, absolutely, you can see that. But personally I don't find it distracting)
2: If you're going to print, a lot of noise also disappears, especially with inkjet. And if you print on canvas, a lot of noise really disappears, because the photo is already going to be softer anyway.
3: Noise only becomes distracting with dark photos. I've often had it happen that I wasn't paying attention for a moment and the camera was still at a very high ISO when I went from inside to outside. And just took photos. But because there is enough light falling on your sensor, there is hardly any noticeable noise at that high ISO.

I use a Nikon Z6ii in M mode, often with auto ISO. The ISO is limited to 12800, previously to 6400. My Nikon guru (Steve Perry) recommends 12800 for this camera. However, I do get nervous when the ISO value reaches 10,000, but I do need this value. This noise is visible in the photo viewer on my laptop. Once in PSE 2020 and/or Topaz, I can often manage to remove the noise.

I never set ISO to automatic because I prefer to be able to make adjustments in between. I also don't want to depend on the automatic settings that can be set on the camera, which means there's a good chance the photo will fail or at least not meet my wishes.

With this article, the A7 is exposed at higher ISO.
With my Dx II and III, 6400 and 12800 are no problem at all

I photograph with a Canon Eos R7 and up to ISO 6400, as far as I'm concerned, it doesn't cause any problems at all in terms of noise, at least in normal lighting conditions.
It becomes a bit different when you start photographing in somewhat darker conditions, because noise at ISO 6400 quickly becomes noticeable then.

What I find strange is that you can't set it lower than ISO 100, while some cameras in automatic mode can go as low as ISO 50.
I remember that in the analogue era, a slide film was rated at ASA/ISO 50 or 18DIN. I would like current cameras to also be able to be set to ISO 50, which is possible with some professional models. There are situations imaginable where you could make very good use of ISO 50.

@Nando, no: in a theater, for example, you do want control over your shutter speed (you want to freeze the dancers/actors in motion, for example) and the light changes suddenly and unpredictably => you lock the shutter speed, the aperture as well (depending on depth of field) and let the camera adjust the ISO to get a good exposure.

setting aperture and shutter speed and ISO to auto is widely used in street photography

Nando, doesn't it simply come down to many of us being too afraid of noise?

I hear many people complain bitterly about their new camera, as if it produced too much noise. I then think it's mainly down to the photographer.
Moreover, noise reduction/removal in editing software has become so good nowadays that noise isn't really a problem anymore.

Nando, so far my upper limit is set to ISO 6400 and with that I very rarely need to correct noise in post-processing. But I will take you up on your suggestion to investigate whether it is possible to go even higher without having to intervene afterwards.

Underexpose by 2 stops. 1600 asa. Push processing in microdolX. Beautiful light, detailed negatives. No DSRL can match it.

A professional photographer with long experience claimed that at elevated ISO (between roughly 100 and 2000 ISO), every camera has a specific ISO value at which noise is present to a minimum.
There should be a list of all camera models regarding this, but I haven't found it so far. Does anyone have experience with this?

People are always looking for a clear-cut rule for 'acceptable noise' but that is actually highly situation-dependent. If your shutter speed becomes too slow so that your noise does become acceptable, you could get motion blur. So use the right shutter speed, the shallowest depth of field that is acceptable and then hopefully the ISO will be acceptable. I think it is a good idea to experiment with this so that later, in a similar situation, you can estimate beforehand how much noise you are going to get. In such situations I personally like to shoot in Manual mode with automatic ISO.
Many cameras have multiple native ISOs and it is worth investigating which of the other ISOs produce the least noise (see also the photonstophotos website). If you still have more noise than you would like and noise-reduction software is not entirely satisfactory, you could consider converting the photo to black and white.