As often in photography, the best ISO setting comes down to a compromise. It also depends on the situation.
For real estate interiors ISO 400 is generally a good choice. This is because it helps you get bright, attractive photographs of your property, while still keeping the image sharp.
When you're choosing the settings to use for your exposure bracketing, there are a few camera settings to consider. The first two are the aperture and shutter speeds you'll use for each shot.
The third setting is the ISO, which is how sensitive your camera sensor is to light.
The reason ISO 400 works so well is because it is low enough to not introduce a lot of noise, but high enough to not require extremely long exposure times when exposing for shadows, and so helping to avoid blurry photos.
ISO is a setting that affects the brightness or darkness of a photo in conjunction with the shutter speed and aperture. The ISO setting you choose will make the camera sensor more or less sensitive to light. Digital cameras these days have ISO options between 100–32,000, or even higher!
If you keep the aperture and shutter speeds the same, changing the ISO will change the brightness of the image. A lower ISO makes the sensor less sensitive, and the image will be darker. Raising the ISO will make the sensor more sensitive, and thus the image will be brighter.
There is a side effect to increasing the sensitivity of the sensor though. If you raise the ISO too much, you will affect the quality of the image. Higher ISOs add more grain or digital noise to the image.
You can adjust the ISO to give you more flexibility with the other settings you use for your photo. For example, when photographing real estate interiors, a higher f-stop is best so that the entire room is in focus. This can mean very long shutter speeds though, especially when you are shooting for the shadows. A higher f-stop means the aperture opening is smaller, which lets less light in.
By raising the ISO a little in this situation, you can shorten the shutter speed while keeping your aperture the same. A good starting point is ISO 400, which will shorten the shutter speed but still keep the amount of noise reasonable.
When ISO 400 won’t work for your needs, trade offs can be made to use a different setting.
If you're finding that the shutter speed you use with ISO 400 is too long for hand held shots, adding a tripod can solve that. This is recommended for real estate photography anyway, and can help you get sharper results. You can also use the self-timer or a remote shutter for your camera to keep those shots with longer shutter speeds crisp.
However, if the shutter speed is just too long for your camera, you can raise the ISO even more. Depending on your camera model and its sensor size, you may be able to raise it quite a bit before the amount of grain becomes unacceptable.
In a very well lit room, lowering the ISO can give you a cleaner image. Very bright rooms look great with ISO 200, and depending on your aperture ISO 100 could work as well. In either case this will increase the shutter speeds, but in a bright enough room it would still be a manageable length, especially if you are using a tripod.