Minimum/Close focus distance of plate cameras

I am aware that the plate cameras can focus to very close distance by extending the bellows.

But, I personally have no experience in shooting with plate cameras with bellow extension so I have no idea about the minimum focus distance with and without bellow extension and the magnification.

Also have no idea about how realistic the portraits will look when shoot at close distance, so I wonder which plate camera/lens can offer the minimum focus distance without unrealistic looks.

Won't this depend on the actual camera model? If it will help, I can check the minimum focus distance on my c1896 E&T Underwood Instanto quarter-plate camera using the original lens, at the weekend?

I am quite curious about the minimum focusing distance without bellow extension of plate cameras of format 9x12 with focal length of 13.5cm if not 6.5x9 with 10.5cm.

Do you mean without bellows extension compensation (because without bellows extension, it won't ever be in focus for anything)? That starts as soon as you focus on anything closer than infinity. The thing is, most people ignore it because at longer distances, the amount of compensation required is extremely minimal, and probably not worth worrying about. The problem with answering your question, is how much loss of light are you okay with before you want to consider bellows extension compensation? Is a third of a stop something that worries you? How about a half stop? What about a whole stop?

The formula is (Bellows Extension distance/Focal length of Lens)^2 = Amount of time needed to add expressed as a multiple of the original time.

In other words: (27/13.5)^2=4
or
27cm (bellows extended to double the length of infinity focus) divided by 13.5cm (focal length of first lens you listed) squared equals 4 (so you'd have to quadruple the time to compensate for this bellows extension). This is also 1:1 magnification, or about where true macro photography begins.

Now, lets say the bellows were only extended to 20.25 cm (or one and a half times infinity focus). (20.25/13.5)^2=2.25. So at that focal length, you'd have to increase the time by slightly more than double (2.25x to be precise).