I have a 27 mm and a 34 mm monitor on my arms. It's exactly as Bravo_83 wrote.
It depends on how far or close you want the monitors and how deep your desk is.
In any case, you have a lot more space on the desk because the feet of the monitors are missing. Once adjusted
I tightened the Allen screws a little so that they don't move so easily.
I'm very happy with it.

Do you mean forwards or backwards?
I have 32 monitors and they can be set up to stand in front of, on or behind the edge of the desk...
(The arms work great)